The correct option is
B a
The words, "a", "an" and "the" are called articles or determiners. They work as adjectives and modify a noun. The word, "the" is used to refer to a specific/ particular noun. It is called a definite article. The words "a" and "an" are used to modify non-specific/ non-particular nouns. These are called indefinite articles.
In the given sentence, the noun being modified is "social animal". The term, "social animal" is used to refer to a creature which greatly interact with other members of its species. It doesn't refer to any specific social animal. As the noun is non-specific, we cannot use "the", which is a definite article. Hence, option C is incorrect.
Using "a" or "an" depends on the sound that begins the next word. If the next word begins with a consonant/ consonant sound, the article "a" is used. If the next word begins with a vowel, the article "an" is used.
In the given sentence, the word following the blank begins with the letter"s", which is a consonant. Hence, the appropriate article is "a". Hence, option A is correct and option B is incorrect.
Option D is incorrect as option A is the correct answer.