Explanation
Correct answer: D.
Choice D is the best answer. The conventions being tested are the use of possessive determiners and the use of plural nouns. The singular possessive determiner "its"—which agrees in number with the singular noun phrase "Mira Nair’s 1991 film Mississippi Masala"—and the plural noun "protagonists" correctly indicate that Nair’s film has multiple protagonists.
Why the other choices are wrong
Choice A
Choice A is incorrect because "it’s" is the contraction for "it is," not a possessive determiner.
Choice B
Choice B is incorrect because the context requires the plural noun "protagonists," not the singular possessive noun "protagonist’s."
Choice C
Choice C is incorrect because the context requires the possessive determiner "its" and the plural noun "protagonists," not the contraction "it’s" or the singular possessive noun "protagonist’s."