Answer:
The answer is "told".
Explanation:
Let's break down the sentence.
Theo, Kate, party are all nouns.
About is being used in a prepositional tense.
Her is a pronoun that is being used to replace Kate's name that would have been duplicated.
Upcoming is an adjective to describe Kate's party- along with surprise.
Accidentally is an adverb (note the "-ly" at the end), and it is modifying the verb, "told". Therefore, accidentally is modifying the verb, told.
