Final answer:
The appropriate word to fill the blank is 'cuántos'.
Step-by-step explanation:
To fill the blank in the given sentence, we need to determine the appropriate form of the adjective 'cuánto' based on the noun it modifies, which is 'libros' (books). Since 'libros' is a masculine plural noun, we need to use the masculine plural form of the adjective, which is 'cuántos'. Therefore, the correct answer is C. Cuántos.
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