Explanation:

Two pure chiral isomers must be optically active. Let's break down the statements to understand why this is the correct answer.

Chirality:
Chirality refers to the presence of a molecule that cannot be superimposed on its mirror image. In other words, a chiral molecule is non-superposable on its mirror image.

Isomers:
Isomers are molecules that have the same molecular formula but different structural arrangements or spatial orientations.

Stereoisomers:
Stereoisomers are isomers that have the same molecular formula and connectivity but differ in the arrangement of atoms or groups in space.

Enantiomers:
Enantiomers are a type of stereoisomer that are mirror images of each other and cannot be superimposed. They have opposite configurations at every chiral center in the molecule.

Diastereomers:
Diastereomers are stereoisomers that are not mirror images of each other and can be superimposed on their mirror images. They have different configurations at one or more chiral centers in the molecule.

Optical Activity:
Optical activity refers to the ability of a substance to rotate the plane of polarized light. Chiral molecules can exhibit optical activity because their mirror images are non-superposable.

Explanation of the Answer:

- Statement a) They must be enantiomers:
This statement is incorrect because two pure chiral isomers can be either enantiomers or diastereomers. Enantiomers are a specific type of stereoisomers, but there are other types of stereoisomers as well.

- Statement b) They must be diastereomers:
This statement is incorrect because two pure chiral isomers can be either enantiomers or diastereomers. Diastereomers are stereoisomers that are not mirror images of each other, but they can still have the same configuration at some chiral centers.

- Statement c) They must be stereoisomers:
This statement is correct because two pure chiral isomers are definitely stereoisomers. Stereoisomers have the same molecular formula and connectivity but differ in the spatial arrangement of atoms or groups.

- Statement d) They must be optically active:
This statement is correct because two pure chiral isomers must be optically active. Chiral molecules have non-superposable mirror images, and as a result, they can rotate the plane of polarized light.

Therefore, the correct answer is option 'D' - They must be optically active.