Final answer:
The subject of this question is Spanish grammar and the correct verb form to use in relation to singular and plural nouns.
Step-by-step explanation:
The correct answer to the question is:
A. costa "¿Cuánto cuesta la corbata?"
The subject in this question is Spanish, and it concerns the comprehension of the grammar rule related to singular and plural forms. In this case, since the question is inquiring about the cost of a tie (la corbata), which is a singular noun, the correct verb form to use is cuesta.
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