Final answer:
The adverb 'cautiously' modifies the verb 'stepped,' describing how Terri Schuester moved over the puddle of melted ice cream.
Step-by-step explanation:
In the sentence 'Terri Schuester stepped cautiously over the puddle of melted ice cream,' the word cautiously is an adverb modifying the verb 'stepped.' It describes how Terri Schuester is performing the action of stepping, indicating that she is doing so with care to avoid the puddle.