ABSTRUSE:

Answer: Option D

ABSTRUSE means something that is hard to understand or obscure.

Among the given options, the word Profound refers to something that is deep or complex in meaning, which closely aligns with the meaning of Abstruse.

On the other hand:

Awful means terrible or very bad – not related.

Irrelevant means not related to the subject – unrelated in meaning.

Shallow means lacking depth – actually the opposite of abstruse.

Therefore, Profound is the most appropriate synonym for Abstruse.