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. "The" is a Definite article.
"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). 
Article "A" takes place with countable nouns in singular numbers that begin with consonants(Eg. A car). 
"The" is the definite article. "The" transpires when we talk about a particular thing. Eg. The movie that we watched was good. Similarly," The" is also used for some proper nouns. Example- The Red Sea, The Nile. Moreover, "The" also applies to things that are only one of a kind. E.g., The Earth, The Sun. 
A consonant is a speech sound that is not a vowel. So we can say that non- vowel sounds are consonants. 
Here, the egg is one and commences with a vowel(E). Therefore it is said as "an egg". Option B is incorrect because "a" takes place with nouns that begin with consonants. Option C is wrong since it has "the" as an article. Option A is the correct answer.