Concept:An adverb modifies a verb, an adjective, or another adverb.
Explanation:The underlined word is "terribly."
In the sentence, "aching" is the verb.
"Terribly" tells us how the arms are aching.
It answers the question: "How are the arms aching?"
Since "terribly" modifies the verb "aching," it is an adverb.
It does not name a person, place, or thing, so it is not a noun.
It does not describe a noun, so it is not an adjective.
It is not a verb form used as an adjective, so it is not a participle.
Answer:C. Adverb