Nouns are words that name people, things, places,
animals, etc.
We form the plural noun by adding an 's' to it.
Option C: 'Chefs' is the plural form of the noun 'chef'. Hence option C is correct.
Option A: 'Chef' is a singular noun that names a professional cook. It's not a plural noun and hence A is wrong.
Option B is wrong because 'chef's' is a singular noun 'chef' in the possessive case (by adding 's to chef).
Option D is wrong because the answer lies in C.