What topic is the scholar currently researching? which word is the interrogative adjective? scholar topic currently what

The interrogative adjective in What topic is the scholar currently researching? is what. It is an adjective because it is modifying the noun topic.  If there was not a noun in the sentence WHAT becomes an interrogative pronoun.  For example

What is that?  There is no noun to modify

D) What.

Explanation: an adjective is a word that describes or modify a noun. There are many different types of adjectives, for example: descriptive, possessive, interrogative, quantitative, demonstrative, etc. An interrogative adjectives are use to form questions, they are whose, what, and which. In the given sentence we can see an example of an interrogative adjective in the word "what" corresponding to option D.