There is only one noun or pronoun in this sentence, it is bag.
Heavy does indeed modify the bag.
Final Answer: b. bag
Let's check!
A. really (really is an adverb, adjectives can't modify adverbs) B. bag (bag is a noun, adjectives modify nouns) C. is (is is a verb, adjectives can't modify verbs) D. that (that is an article, adjectives can't modify articles)
