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