What Is the Key Reason That Often Only One Isomer

What is the key reason that often only one isomer of a pair of enantiomers is biologically active?

A) The functional groups in isomers may be found on the same or opposite sides of a double bond.
B) Isomers of an organic molecule may have either branched or linear structures.
C) One isomer may take up more volume.
D) One isomer may be more polar than the other.
E) Only one isomer may bind to specific target molecules in an organism.