The correct option is "interrogative adjective".
Interrogative adjectives are words that modify nouns or other terms inside a question. They can be compared to interrogative pronouns, but the difference is that interrogative adjectives don't stand on their own and they always act in relation to another term.
In this example the word "What" acts as an interrogative adjective that modifies the term "on Earth".
If the sentence would've been "What is causing all the excitement today?", then the word "what" would be functioning just as an interrogative pronoun.
Hope this helps!