(b) : Chargaff’s rules are applicable only for double stranded $DNA$ molecule. These are not applicable for single stranded $DNA$ or $RNA$ molecules. Chargaff’s rules state that $DNA$ helices contain equal molar ratios of $A$ and $T, G $ and $C$. This is because in a ds $DNA$ molecule, complementary base pairing occurs between $A$ and $T$, and $C$ and $G$ base pairs. This complementary base pairing is not possible in case of single stranded $RNA$ molecule. Thus, Chargaff’s rules are not applicable to $RNA$.