Explanation: Transformer coupling offers several key advantages over resistance-capacitance (RC) coupling in audio frequency amplifiers. Options A and B describe genuine advantages. * **A) Greater gain per stage:** Transformers can be used to step up voltage, providing higher gain per stage. They also facilitate better power transfer between stages due to their impedance transformation properties. * **B) Can provide necessary impedance matching:** This is a primary strength of transformer coupling. Transformers efficiently match the high output impedance of one stage to the lower input impedance of the next, or to a load, maximizing power transfer and reducing signal loss. Since A and B are both advantages, C is also an incorrect choice as it reiterates actual advantages. **D) A transformer is expensive, heavy, and bulky:** This describes *disadvantages* of transformer coupling, particularly at audio frequencies where larger core materials are needed. Since the question asks "Which is *not* the principle advantage," this statement, being a disadvantage, correctly identifies something that is *not* an advantage.