Here, "a clear picture" is the subject which is Singular. So, we need to use Singular verb "has" instead of plural verb "have". Use of "have" violates S-V agreement. Hence, Choice B and D are out. Answer choice C and E are out because of the use of wrong tense. The sentence is in present perfect tense while Choice E uses past perfect and Choice C uses simple past tense. Hence, A is the answer.

NandishSS wrote:Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture has emerged of the patterns and causes of ocean currents.

A.a reasonably clear picture has emerged of the patterns and causes of ocean currents.
B. a reasonably clear picture of the patterns of ocean currents and their causes have emerged.
C. a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
D. there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them.
E. there had emerged a reasonably clear picture of the patterns of ocean currents and their causes.

OA:A

Source:GMATPrep EP2

for some time indicators such as "in" and "for", both simple past and present perfect are fit, depending on logic. "in' and 'for" can show a period in the past, and, so, past forms are used, or a period which continue until present, and, so, present perfect is used. this is basic knowledge explained in grammar books. time indicators and contexts are focus of discussion of tenses.
I learned gmat this morning. now is afternoon. this morning is past. past simple is used
I have learned gmat this morning. now is in this morning. this morning is a period from a point of past to now.
I have learn gmat for many years, but only yesterday, I passed it.
this is correct.
so, i think that "emergED" , a past verb, is good in C. C is wrong because of " cause of them". this phrase is wordy and change the meaning

GMATGuruNY wrote: ↑

Mon May 22, 2017 2:16 am

NandishSS wrote:Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture has emerged of the patterns and causes of ocean currents.

A. a reasonably clear picture has emerged of the patterns and causes of ocean currents.
B. a reasonably clear picture of the patterns of ocean currents and their causes have emerged.
C. a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
D. there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them.
E. there had emerged a reasonably clear picture of the patterns of ocean currents and their causes.

In B and D, picture (singular) does not agree with have emerged (plural).
Eliminate B and D.

C: Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture emerged of ocean currents.
Here, the meaning conveyed by the red portion (a picture of ocean currents emerged ONLY IN THE LAST HALF CENTURY) contradicts the meaning conveyed by the blue portion (navigators have known FOR THOUSANDS OF YEARS that the ocean has variable currents).
Eliminate C.

In E, it is not crystal clear whether their serves to refer to patterns or to currents.
Since A avoids this ambiguity and is free of errors, eliminate E.
Also, there is no justification for the usage of the past perfect (had emerged).

The correct answer is A.

Hi Mitch! @GMATGuruNY
Is C a run on sentence? "...a reasonably clear picture emerged of ocean currents, their patterns and the causes of them." tks!