Elephants observed in Kenya used different trumpeting noises depending on the purpose of their call: some sounds were intended as a warning call, while others to express emotion and to intimidate predators.
A. to express emotion and to intimidate
B. intended to express emotion and intimidating
C. expressed emotion or intimidated
D. were to express emotion or intimidate
E. intended the expressing of emotion or intimidating

(C) The answer corrects the parallelism and diction errors in the original sentence. The sentence describes a situation: in Kenya, elephants were observed making various kinds of calls. The underlined part of the sentence contains a list of two kinds of calls. Because all of the elements in a list must be in the same form, we can eliminate choice (B), whose verbs are in different forms: ‘express’ and “intimidating.” Furthermore, because the sentence implies that each call has a single purpose, the proper linking word to use here is or, not and: “express emotion or intimidate predators.” Choices (C), (D), and (E) all contain this construction. Of these remaining choices, (C) is most concise and most grammatical: “others expressed emotion or intimidated predators.” Choice (D) omits the word “intended”, which is necessary to maintain parallelism with the phrase “were intended” that is found in the non-underlined part of the sentence (it should read “were intended to express emotion or intimidate predators”). Choice (E) is too wordy; furthermore, it fails to include the preposition “of” in the prepositional phrase “intimidating [of] predators,” which is necessary to maintain parallelism with the phrase “expressing of emotion” introduced by this answer choice. Choice (C) corrects the errors in the original sentence and avoids introducing new errors. (C) is the best choice.
----------
The answer should be parallel with "some sounds were intended as a warning call".. so option A is correct.

Choice (A) is not parallel because it uses the present tense while the non-underlined portion of the sentence uses the past tense ("some sounds were intended as a warning call"). Choice (C) uses the correct verb forms and has no other grammatical errors.