Explanation:

Optical isomerism, also known as enantiomerism, occurs when a molecule has a chiral center, or an atom that is bonded to four different groups. In hydrocarbons, the carbon atom serves as the chiral center.

Chiral Center:
A chiral center is a carbon atom bonded to four different groups, where these groups can be different atoms or groups of atoms. A chiral center is necessary for a molecule to exhibit optical isomerism.

Minimum Number of C Atoms:
In order for a hydrocarbon to have a chiral center, it must have at least four carbon atoms. This is because a chiral center requires three carbon atoms to form the tetrahedral geometry around the central carbon atom, and an additional carbon atom must be bonded to the central carbon atom in order to have four different groups.

Explanation of Options:
a) 4 C atoms: This is the minimum number of carbon atoms required for a hydrocarbon to exhibit optical isomerism. Therefore, this option is correct.

b) 5 C atoms: This is more than the minimum number of carbon atoms required for a hydrocarbon to exhibit optical isomerism. Therefore, this option is incorrect.

c) 6 C atoms: This is more than the minimum number of carbon atoms required for a hydrocarbon to exhibit optical isomerism. Therefore, this option is incorrect.

d) 7 C atoms: This is more than the minimum number of carbon atoms required for a hydrocarbon to exhibit optical isomerism. Therefore, this option is incorrect.

Conclusion:
The minimum number of carbon atoms required for a hydrocarbon to exhibit optical isomerism is 4. Therefore, the correct answer is option 'D'.

C is correct.