Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of punctuation within a sentence. The comma after “swimming” pairs with the dash after “ciliates” to offset the supplementary element (“widely…ciliates”) that interrupts the main clause of the sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect. Adding the verb “is” here creates an ungrammatical sentence structure with two main verbs (“is” and “bestows”) without appropriate punctuation and/or conjunction.
Choice B
Choice B is incorrect because it fails to offset the supplementary element with appropriate punctuation. Furthermore, adding the verb “is” here creates an ungrammatical sentence that has two finite verbs (“is” and “bestows”) without appropriate punctuation and/or conjunction.
Choice C
Choice C is incorrect because it fails to offset the supplementary element with appropriate punctuation. Furthermore, adding the verb “has been” here creates an ungrammatical sentence that has two finite verbs (“has been” and “bestows”) without appropriate punctuation and/or conjunction.