Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of finite and nonfinite verb forms within a sentence. The nonfinite present participle “forcing” is correctly used to form a participial phrase that supplements the main clause “those...cover,” describing the effects on monkeys of the lack of food sources.
Why the other choices are wrong
Choice A
Choice A is incorrect because the finite present tense verb “forces” can’t be used in this way to supplement the main clause (“those...cover”).
Choice B
Choice B is incorrect. While the nonfinite to-infinitive “to force” could be used to form a subordinate clause that supplements the main clause (“those...cover”), to-infinitives conventionally express purpose, and nothing in the sentence suggests that the food sources become unavailable for the purpose of forcing monkeys to hunt marine animals.
Choice D
Choice D is incorrect because the finite past tense verb “forced” can’t be used in this way to supplement the main clause (“those...cover”).