Adjectives modify nouns and pronouns.
The adjective in this sentence is heavy.
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)