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 'box').
Option A: 'Boxes' is the plural form of the noun 'box'. Hence option A is correct.
Option B: 'Box' is a singular noun that names an object. It's not a plural noun and hence B is wrong.
Option C is wrong because 'boxed' is not a noun, but a verb.
Option D is wrong because the answer only lies in A.