The correct option is
B bags
Nouns are words that name people, things, places,
animals, etc.
Nouns function as the subject, object and so on in a sentence. As a subject it performs an action.
Option C: The word 'bags' names an object in general. It functions as the object of the given sentence, receiving the action 'pack'.
Hence option C is correct.
Option A: 'Pack' is a verb that expresses an action. It's not a noun and hence A is wrong.
Option B: 'Carefully' is an adverb that modifies the verb 'pack'. Adverbs describe/modify verbs, adverbs and adjectives. It's not a noun and hence B is wrong.
Option D is wrong because the answer lies in C.