Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of possessive nouns. The singular possessive nouns "rock’s" and "rap’s" correctly indicate that the chorus-to-verse ratios belong to the genres of rock and rap.
Why the other choices are wrong
Choice A
Choice A is incorrect because the context requires the singular possessive nouns "rock’s" and "rap’s," not the plural nouns "rocks" and "raps."
Choice C
Choice C is incorrect because the context requires the singular possessive noun "rock’s," not the plural noun "rocks."
Choice D
Choice D is incorrect because the context requires the singular possessive noun "rock’s," not the singular noun "rock." This choice incorrectly suggests that there was a single chorus-to-verse ratio for rock and rap together, whereas the ratio was calculated for each genre individually.