One Word Substitution

 Gratis is correct. It is from the Latin which means "in gratitude" or something given in gratitude. 

Impassable describes something that cannot be traveled through or across.

A philistine is a person who is hostile or indifferent to culture and the arts.

Omniscient is derived from Latin roots meaning 'all-knowing'.

A zoo is a facility in which animals are confined within enclosures, displayed to the public, and in which they may also be bred.

A widower is a man whose wife has died and who has not remarried.

Edible refers to something that is safe or fit to be eaten.

Chronologically means arranged in the order of time.

A dentist is a medical professional qualified to treat diseases and conditions of the teeth.

A crew is a group of people who work on and operate a ship, aircraft, or other vehicle.

 A cannibal is one who eats their own kind, but is most often ascribed to humans who will eat other humans.

A nostrum is a medicine prepared by an unqualified person, especially one that is not considered effective or has no demonstrable value.

Immortal means living forever or never subject to death.

Incredible describes something that is impossible or very difficult to believe.

An aviary is a large cage or building for keeping birds. Other options like zoo are too general, while visible and bloody are unrelated adjectives.

Invincible is derived from Latin roots meaning not able to be overcome. Patriotic, theist, and zoo are clearly incorrect definitions.

A biped is an animal that uses two legs for walking. The other options are unrelated terms.

A mortuary is a room or building where dead bodies are kept before burial or cremation. Other options are irrelevant nouns.

Taciturn describes a person who is reserved or habitually silent. Suicide, heretic, and zoo do not fit the definition.

An orthodontist is a dentist who specializes in correcting the alignment of teeth. Botanist is for plants, and the other options are nonsense in this context.

Geology is the scientific study of the Earth, its materials, and the processes acting upon them. The other options refer to animals or religious beliefs.

A braggart is a person who boasts about their achievements or possessions. Ascetic, atheist, and bloody do not match this definition.

 Erudite means having a wide knowledge gained from reading.

 Correct answer is ambivert.

 Placate means to stop from being angry, i.e. to pacify.