An unattractive environment can evoke feelings of dislike, aggression, and hostility, whereas an attractive environment induces happiness, warmth, and security. a) definite articleb) indefinite articlec) not an article

b) Indefinite article (an)

Explanation:

'A' and 'An' are recognized as the two indefinite articles as these are invariable and employed only with a singular noun. It is used to denote something for the first time or refer to a specific person or a member of a group or class. Since the given sentence has usage of "an" which is used here to specify the distinction between an "unattractive environment" and an "attractive environment". Thus, the above definition is being justified so, it could be treated as an indefinite article.