How can you identify chiral carbon and optically active compound by seeing the molecule?

The correct option is A If all the atoms or groups attached to carbon are different
Chiral compounds can be identified by looking at the carbons in the compound. If there are $4$ different groups attached to the carbon then it is chiral. If there are $2$ or more of the same groups attached then the compound is achiral. (Each carbon in the compound must be looked at to determine if it is chiral or achiral).
Hence option A is correct.