Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of plural and possessive nouns. The singular possessive noun “playa’s” and the plural possessive noun “rocks’” correctly indicate that the sediment is that of one playa (the Racetrack Playa) and that there are multiple rocks that have mysteriously migrated across the sediment.
Why the other choices are wrong
Choice A
Choice A is incorrect because the context requires the singular possessive noun “playa’s” and the plural possessive noun “rocks’,” not the plural noun “playas” and the singular possessive noun “rock’s.”
Choice B
Choice B is incorrect because the context requires the plural possessive noun “rocks’,” not the plural noun “rocks.”
Choice D
Choice D is incorrect because the context requires the singular possessive noun “playa’s,” not the plural possessive noun “playas’.”