As given in the line "I read somewhere that there was once .... Aberdeen terrier", the noun 'terrier' is mentioned for the first time in the passage. It's a non-specific noun, hence an indefinite article is used. Option B is an indefinite article used for nouns which begin with a vowel. When an adjective is placed before the noun, we consider the initial letter of the adjective, like in the 'Aberdeen terrier' the vowel 'A' determines the correct option 'an', hence option B is correct. Option A is incorrect because it is used to modify nouns which begin with a consonant. Even though the noun terrier begins with the consonant 't', we consider the first letter of the adjective Aberdeen, which is a vowel, that modifies terrier. Option C is incorrect because the noun is being mentioned for the first time, and is not specific. Option D is incorrect because Aberdeen terrier is a non-specific, countable noun and we need an article to modify it since we are not referring to it in a general sense.