In the above given sentence, nouns are “Colony”, “Ants”, “Backyard”. The collective nouns can be basically written in the form “a swarm of bees”, “a group of people”, “a band of musicians”, etc. Option A, “Colony”, a set of ants is in fact called a Colony, Hence , This is the correct option.

Option B, “Ants” implies a plural form while the collective nouns are usually in singular form, also, ants are the entities that make up a set, hence this is an incorrect option.

Option C, “Backyard” refers to a place or a location and not a set of entities, hence , this is an incorrect option.

Option D, “Colony, Ants” includes “Ants” which is not a collective noun, as discussed above, Hence it is incorrect as well.