Complete the eavesdropper's whispered question to their friend by selecting the correct option.

"They've been arguing loudly for twenty minutes, but I missed the beginning! _____ are they actually arguing _____?"

The correct answer is What / about.

The verb "argue" pairs with the preposition "about" when discussing the topic of the argument. In a question asking for that topic, we start with What and end with the preposition about.