Answer:
Slowly.
Step-by-step explanation:
Modifiers are words, phrases, or clauses that modify other words in a sentence. This means that they help give 'extra' detail about the word they are modifying. Modifiers can either be an adjective or an adverb, depending on the word they modify.
In the given sentence "I slowly ate the pizza; it took me 20 minutes to finish a slice", the modifying word is "slowly". This adverb modifies the verb "ate", thereby making it an adverbial clause. Adjective modifiers modify nouns while adverb modifiers modifier verb, another adverb, or an adjective.
Thus, the modifier in the sentence is "slowly".