Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is subject-verb agreement and agreement between nouns. The singular verb “is” and the singular noun “version” both agree in number with the relative pronoun “which.” In this context, “which” functions as a singular subject because it refers to the singular noun “the Nerf football.”
Why the other choices are wrong
Choice A
Choice A is incorrect because the plural verb “were” doesn’t agree in number with the singular noun phrase “the Nerf football” that it’s modifying.
Choice B
Choice B is incorrect because the plural verb “are” and the plural noun “versions” don’t agree in number with the singular noun phrase “the Nerf football” that they’re modifying.
Choice C
Choice C is incorrect because the plural verb “were” and the plural noun “versions” don’t agree in number with the singular noun phrase “the Nerf football” that they’re modifying.