Final answer:
The correct word to fill in the blank is 'Cuántos'.
Step-by-step explanation:
The correct word to fill in the blank in the question '¿Cuánto (how much) cuestan los libros?' is C. Cuántos. The word 'libros' is in plural form, so we need to use 'cuántos' which means 'how many.' 'Cuánto' is used for singular nouns, 'cuánta' is used for feminine singular nouns, and 'cuántas' is used for feminine plural nouns.
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