Final answer:
An enantiomer is a stereoisomer that is a non-superimposable mirror image of another molecule; it requires a chiral center with four different substituents for its existence.
Step-by-step explanation:
The enantiomer of a drug like methamphetamine is a stereoisomer that has the same molecular formula as the original substance but is a non-superimposable mirror image. Enantiomers are a type of isomer where the molecules are mirror images of each other but cannot be superimposed onto one another, much like left and right hands. This definition corresponds to option (a), stating an enantiomer is a 'Stereoisomer with the same molecular formula'. This is true because enantiomers must have the same molecular formula and physical and chemical properties, except for their interaction with polarized light and other chiral substances. Notably, for a molecule to have enantiomers, it must have at least one carbon atom with four different atoms or groups attached to it, rendering it a chiral center (Answer d is correct).