Nouns are words that name people, things, places,
animals, etc.
When a noun ends with an 'x', we form the plural noun by adding an 'es' to it ('x' in 'fox').
Option C: 'Foxes' is the plural form of the noun 'fox'. Hence option C is correct.
Option A: 'Fox' is a singular noun that names an animal. It's not a plural noun and hence A is wrong.
Options B and D: ''Fox's' is a singular noun (fox) in the possessive case ('s refers to possession)
Foxes' is a plural noun in the possessive case. But, the sentence does not indicate any possession.
Hence these options are wrong.