Correct Answer: (d) A; an
Solution:The 'building' mentioned in the given sentence is non-specific and we generally use the indefinite article 'a' before any non-specific or non-particular noun. However, the article 'an' is used before words with starting a vowel(A,E,I,O,U) for example, an educated person, an imminent danger, enormous collection(in the given sentence). Hence, 'A, an' is the most appropriate answer.