What is descriptive fair use in trademark law?

Select one:

a. It is a four-factor test that balances public and private interests.

b. It allows producers to use other producers' marks as part of comparative advertising.

c. It allows a producer to use a word that is a trademark when the word is used in its nontrademark sense to describe the producer's own goods.

d. It allows anyone to use a mark that has become generic over time.