Final answer:
The correct indefinite article to complete the sentence is 'un'
Step-by-step explanation:
The sentence is asking for the correct indefinite article to complete the sentence 'Necesito comprar (I) libro.' The indefinite articles in Spanish are 'un' for singular masculine nouns and 'una' for singular feminine nouns. Since the word 'libro' is a masculine noun, the correct indefinite article would be 'un.'
Learn more about indefinite articles in Spanish