The correct answer is C. Completely
Step-by-step explanation:
In grammar, adverbs are a type of words that act as a modifier of verbs which means they describe the way an action occurs. In the English language most adverbs can be identified because they end in "-ly" such as slowly, enthusiastically or carefully, although there are adverbs such as now, often or here do not follow this rule. Considering this, in the case of the sentence "He completely answered all of the questions before leaving the room" the word that is an adverb is "completely" because this describes or modifies the action "answer" and besides this, this end in "-ly" which are features of adverbs.