Final answer:
The correct indefinite article to complete the sentence 'taza de café' is 'una'.
Step-by-step explanation:
The correct indefinite article to complete the sentence 'taza de café' in Spanish is una.
The word 'taza' is a feminine noun, and in Spanish, feminine singular nouns generally use the indefinite article 'una'.
In this case, 'taza' is singular, so we use the singular form of the article, which is 'una'.
Learn more about Indefinite articles in Spanish