While I believe that both A and D are correct, the best choice here would be A
This means that he feels very drugged as if he had just taken an opioid or something. He hears a nightingale singing saying that the bird is spreading its happiness too much. He is also saying that he too is happy because he can hear the nightingale sing somewhere amidst the forest.
I believe to is your answer.
Hope this helps!
Stay smart,
~{Dunsforhands}
