Answer: A: He is losing his grip on reality and/or feeling guilty.
Step-by-step explanation:
Macbeth's "encounter" with a ghost is not actually reality. It's all an illusion, due to the work of his mind. He has lost his sanity and is guilty because deep down he is troubled by the killing, though on the surface it may seem he enjoys it. This clearly marks the decline of his power.
Hope this helps,
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