Organic Chemistry Fundamentals

Crystallography is an unambiguous method for determining molecular geometry, the proviso being that single crystals of the material must be available and the crystal must be representative of the sample. Highly automated software allows a structure to be determined within hours of obtaining a suitable crystal.

Azulene is an organic compound and an isomer of naphthalene. 

Alkenes and cycloalkanes have the same molecular formula (C_nH_2n). Since cycloalkanes do not behave as unsaturated hydrocarbons, alkenes and cycloalkanes are termed functional isomers. For instance, propene is isomeric with cyclopropane (or trimethylene).

Nonacosane is a straight-chain hydrocarbon. It has 1,590,507,121 constitutional isomers. Nonacosane has been reported to be a component of a pheromone of Orgyia leucostigma, and evidence suggest it plays a role in the chemical communication of several insects, including the female Anopheles stephensi mosquito.

2,2-Dimethylbutane, trivially known as neohexane, is an organic compound with formula C₆H₁₄ or (H₃C-)₃-C-CH₂-CH₃. It is therefore an alkane, indeed the most compact and branched of the hexane isomers — the only one with a quaternary carbon and a butane (C₄) backbone.

Thietane is a heterocyclic compound containing a saturated four-membered ring with three carbon atoms and one sulphur atom.

Thiete is a heterocycle. It is more commonly encountered not on its own, but in anellated derivatives, several of which have been synthesized. Thietes are generally not very stable.

Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric strain barrier to rotation is high enough to allow for the isolation of the conformers.

Conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds. Such isomers are generally referred to as conformational isomers or conformers and specifically as rotamers when they differ by rotation about only one single bond.

Chair conformations are the most accurate representations of how cyclohexane rings are actually oriented in space. They appropriately reflect the angles between the carbons in the ring and the positions of the groups on each carbon in the ring.

Axial chirality is a special case of chirality in which a molecule does not possess a stereogenic center (the most common form of chirality in organic compounds) but an axis of chirality - an axis about which a set of substituents is held in a spatial arrangement that is not superposable on its mirror image. Axial chirality is most commonly observed in atropisomeric biaryl compounds wherein the rotation about the aryl-aryl bond is restricted.

This statement is correct, because the molecule as whole is chiral, since this molecule and its mirror image are non-superimposable.

1-methoxypropane and 2-methoxyprpane are chain or position isomers, a racemic mixture shows no optical activity and m-chlorobromobenzene and m-bromochlorobenzene represents the same compound, so the statement A, B, and C all are incorrect. Thus, this option is correct.

This option is correct because 1, 2-propadiene does not exhibits optical isomerism due to it contains axis of symmetry.

Since, diastereomers may or may not be chiral,  they may or may not be optically active. Thus, this option is correct.