Final answer:
The term that best describes the italicized word is interrogative adjective.
Step-by-step explanation:
The term that best describes the italicized word in the sentence is interrogative adjective. In the sentence, 'What on Earth is causing all the excitement today?' the word 'what' is an interrogative adjective because it is used to ask a question and modify the noun 'excitement'. It is asking for the identity of the cause of the excitement, making it an interrogative adjective.
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