Bunuel
Northern pike – also known as jackfish – are a cunning and aggressive fish, propelled by fast-twitched S-shaped movements as they emerge from sedentary hiding places to quickly accelerate and track down their unsuspecting prey.
A. Northern pike – also known as jackfish – are a cunning and aggressive fish, propelled by fast-twitched S-shaped movements as they emerge
B. The northern pike – also known as jackfish – is a cunning and aggressive fish, propelled by fast-twitched S-shaped movements as it emerges
C. Northern pike – also known as jackfish – are cunning and aggressive fish that are propelled by fast-twitched S-shaped movements and emerging
D. The northern pike – also known as jackfish – is a cunning, aggressive fish that is propelled by fast-twitched S-shaped movements, emerging
E. The northern pike – also known as a jackfish – is a cunning and aggressive fish and is propelled by fast-twitched S-shaped movements while it emerges
VERITAS PREP OFFICIAL SOLUTION:This problem features several singular vs. plural (with both pronouns and verbs) decision points, and relies on a unique structure to make those decisions tricky. As often occurs on the GMAT, however, the testmaker has left a clue for those who recognize that "The Whole Sentence Matters." Note the fixed "their" - a plural pronoun - three words from the end of the sentence and far from the underline. This means that "northern pike" are meant to be treated as plural requiring that verbs that describe them must be "are" and not "is" (eliminating B, D, and E), which all use "is a cunning...") and that pronouns that describe them must be "they" or "their," eliminating B and E which each use "it emerges."
From there, note that choice C uses an improper verb tense ("and emerging"), which creates an illogical meaning for the sentence (everything else in the sentence is in an indicative "these are properties of northern pike" manner, while "are emerging" refers to a current event).
So choice A, which uses proper pronouns and verbs, is correct.
But wait: how does "a cunning and aggressive fish" match with everything else being plural? This is a classic testmaker device, finding unique applications of proper grammar that strike the ear in a funny way. What's occurring there is that the sentence is taking the individual fish ("northern pike are") and defining them as a species "a cunning and aggressive fish." Note that the GMAT has often done this very thing, using the fact that with animals the singular ("the lion" or "the dolphin," for example) can stand in to describe the entire species. This also extends to groups of people ("Norwegians are a hearty people"), where a sentence can treat the people as individuals (plural) and as a group (singular) when discussing group characteristics.
Even more importantly, recognize this about GMAT Sentence Correction: you can only control the decisions that the problem allows you to make. The testmaker intentionally finds unique sentence structures and cases of diction so that often you won't love the answer you pick. But on hard questions your job is to accept those seemingly-strange things that you cannot control and make the decisions that you're able to make.