A proposal has been made to trim the horns from rhinoceroses to discou

First Glance

The sentence contains a semicolon but the underline is limited only to the second half. DO read the first half, as it will provide context for the intended meaning.

Issues

(1) Meaning, and

You might spot this in the original sentence or via a direct comparison of answers (A) and (B):

(A) tourists will continue to visit parks and see rhinoceroses
(B) tourists will continue to visit parks to see rhinoceroses

The X and Y structure in answer (A) does not require the two parallel portions to have anything to do with one another. Consider this example: tourists will continue to visit the pyramids in Egypt and see the Taj Mahal in India. Tourists will continue to do these two things, but the X and Y portions don't have anything to do with each other.

Similarly, in the original sentence, if the actions visit game parks and see rhinoceroses are connected by the word and, then they are completely separate. This doesn't make logical sense. The tourists visit the parks in order to see the rhinoceroses. Eliminate answers (A) and (D).

(2) Pronoun: theirs; one

Some answers use the pronoun their while others use the animals' instead. Other answers change rhinoceroses to one.

The pronoun their is ambiguous; it could refer to rhinoceroses or tourists. Consider this example: these tourists will travel to foreign countries and meet new people after their visas are approved. Whose visas need to get approved? The tourists, not the new people ─ the tourists are the ones who are traveling.

Because their could refer to tourists (it should refer to rhinoceroses), answer (A) is ambiguous. Answer (B) repeats this error; eliminate both.

Answer (B) and (E) replace rhinoceroses with the pronoun one. First it's ok to use one to refer to a plural noun; this construction just means one of those. One of what, though? Logically, one rhinoceros makes sense, but maybe the question is whether tourists will continue to see poachers after the poachers have been discouraged from killing the rhinos ─ that's also a reasonable interpretation. The word one, then, is ambiguous. Eliminate answer (B) and (E).

(3) Verb: are

A vertical scan of the end of each answer choice reveals a split between present tense are and past perfect have been. Are both acceptable?

Verb tenses convey a time frame for an action and indicate when different actions happen relative to one another. In this sentence, the trimming of the horns must happen before a possible visit by tourists, so the simple present tense is inappropriate. The present perfect have been properly indicates that, at the time of a potential tourist visit, the horns must already have been trimmed. Eliminate answers (A), (B), and (D).

The Correct Answer

Correct answer (C) fixes the initial pronoun error by replacing their with the animals'. Furthermore, the sentence changes the X and Y construction to one with a more logical meaning: the tourists visit parks to see rhinoceroses.

Miscellaneous

Nowadays, whether and if are mostly used interchangeably. Formally, though, whether is used to indicate the meaning whether or not: Tell me whether you plan to go to the movies ─ I want to know whether you do or whether you don't. On the other hand, if is used for if-then (conditional) clauses: If you buy me an ice cream, I'll be happy.

To date, no published official questions have used if in a correct answer choice when the meaning technically matches the word whether. The official answer explanations, however, don't actually mention this issue. If you need to guess, don't pick an answer that uses if when the meaning is really whether or not.