Diversity in Living Organisms

Protochordates are closely related to vertebrates. These lack some of the peculiar characteristics of chordates like the true skull, brain, skeleton. These are invertebrates. This group is composed of the phylum Hemichordata and the subphylum Urochordata and Cephalochordata.
So, the correct answer is option D.

Amphioxus or lancelets comprise approximately of 32 species of fish-like marine chordates. In the lancelet, there is a nerve cord above the notochord, but no cranium or vertebrae or eyes.
So, the correct answer is option A.

Buccal diverticulum is a short and thick-walled projection which extend forwards through the proboscis stalk into the proboscis coelom. This is a hollow tube like projection. This is a tubular outgrowth from the mouth cavity forward into the proboscis.The wall is composed of a single layer of tall and vacuolated endodermal cell. Hemichordata is known as half chordate because of the buccal diverticulum. The structure resembles like a rudimentary notochord present in the dorsal or back side and supporting axis for primitive vertebrates. 

Early developmental stages of Balanoglossus possess a spacious coelom, lined with peritoneum. It is enterocoeles in origin, as the adult condition is attained, the coelomic epithelium gives rise to the connective tissue and muscle fibers. Hence, the coelomic cavity is obliterated. The coelom as a whole is divided into three parts. These include one cavity in proboscis, (protocoel), two in the collar, (mesocoel) and two in the trunk (metacoel).

A tornania is the planktonic larva of some species of Hemichordata. It is very similar in appearance to the bipinnaria larvae of starfishes, with convoluted bands of cilia running around the body. Hemichordata is a phylum of marine deuterostome animals.

Protochordates are an informal category of animals (i.e., not a proper taxonomic group). This group has the animals that possess some but not all characteristics of chordates. They have pharyngeal gill slits and a dorsal nerve cord, however, they lack vertebral column and paired appendages.

Hemichordates share some but not all of the typical chordate characteristics and hence, are not true chordates.
There is a rudimentary structure in the collar region, the stomochord, that is similar to a notochord. There is a dorsal nerve cord, in addition to a smaller ventral nerve cord.

Branchiostoma is one of the few living genera of lancelets. They do not have a true head, but merely a mouth adjacent to the gill slits with the slightly enlarged anterior end of the dorsal nerve cord above and in front of them. In Brannchiostoma, the notochord is extended up to the head and is retained throughout its life.

Balanoglossus is an ocean-dwelling acorn worm genus. They possess a long protrusible proboscis.
The tentacular apparatus, which is present in some hemichordates is of lysosomal (collar).
Balanoglossus also possess the trunk in which the gonads are present.

Balanoglossus was earlier placed in the group of the Hemichordates. But now like the other doubtful Hemichordates, such as Cephalodiscus and Rhabdopleura, Balanoglossus are now classified under non-chordates after echinoderms. It is because their notochord is doubtful. They have a short notochord which is confined to proboscis and non homologous with that of chordates.

Amphioxus also called as lancelets or cephalochordates form one of the three chordate subphyla. Amphioxus possess typical chordate characters, such as a dorsal hollow neural tube and notochord, a ventral gut and a perforated pharynx with gill slits, segmented axial muscles and gonads, a post-anal tail, a pronephric kidney. Hence, option B is correct.

The unpaired 'Hatscheck's nephridium' is the largest and most anterior of the nephridia in the body of the Branchiostoma. It is a single tubule which lies on the left side of the notochord running parallel to the left anterior aorta. It stretches from the anterior vestibule to a point immediately posterior to the velum where it opens into the endodermal pharynx. 

Hemichordata closely relates the Chordates. It contains marine deuterostome animals. They have a tubular structure known as stomochords. Stomochords arise in embryonic development as an out-pocketing from the roof of the embryonic gut anterior to the pharynx. In adults, they extend dorsally from the pharynx into the proboscis and serve to communicate with the oral cavity. Their walls are composed primarily of epithelial cells, but ciliated and glandular cells are also present.Balanoglossus belongs to the Hemichordata phylum. So, it has buccal diverticulum.

Protochordates are closely related to vertebrates. These lack some of the peculiar characteristics of chordates like the true skull, brain, skeleton. These are invertebrates. 
So, the correct answer is option A.

Balanoglossus belongs to subphylum Hemichordata, which have a short notochord present in anterior part of the body but it is only present in the larval stage.