The correct answer is D. Profound.
Abstruse means difficult to understand, especially because of its abstract nature. It is often used to describe ideas or concepts that are complex or difficult to grasp.
Awful means extremely bad or unpleasant. It is often used to describe things that are terrible or disgusting.
Irrelevant means not connected with or relevant to the matter in question. It is often used to describe things that are not important or do not matter.
Shallow means not deep or profound. It is often used to describe things that are superficial or not serious.
In the context of the question, the correct answer is D. Profound. This is because abstruse is often used to describe ideas or concepts that are complex or difficult to grasp. These ideas or concepts can be profound, meaning that they are deep or meaningful.