detailed solution

Correct option is A

Option A is the correct answer. In simple words, we can say that articles mean the words - A, An, and The. They are placed before a noun to show whether the noun referred to in a sentence is specific or general. A and An are considered Indefinite articles. "An" is used before nouns in singular numbers that start with vowels{A, E, I, O, U}. It can also occur before countable nouns in the singular number with a vowel sound(Eg. An hour). 
Here, "an ant" is singular and commences with the vowel sound "a". Thus it is said as "an ant". Option B is incorrect because "a" takes place with nouns that begin with consonants. Option C is wrong because "the" article transpires when we talk about a particular thing.