Final answer:
The appropriate word to fill in the blank is 'cuánto'.
Step-by-step explanation:
The appropriate word to fill in the blank in the question '¿cuánto cuestan los libros?' is A. Cuánto. This is because 'cuánto' is the masculine singular form of the word 'cuánto', which is used to ask about the quantity or price of a singular masculine noun, such as 'libros' which means 'books'.
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