B is ungrammatical ("agree that is caused...").
The "in burning fossil fuels" in A and in the original is unidiomatic.
D and E are more convoluted than necessary, though grammatical.
C is the the most natural choice for me.
This also would be good:  a phenomenon that most scientists agree is caused
by human beings burning fossil fuels,

On Mon, Feb 26, 2018 at 3:56 PM, Jessica Thomasson <
[log in to unmask]> wrote:

> Answer C sounds most natural to me.
>
>
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> On Mon, Feb 26, 2018 at 3:53 PM, John Crow <[log in to unmask]> wrote:
>
>> A former student of mine is studying for the GMAT and is struggling with
>> the sentence structure section.  She sent me a sample from the practice
>> test.  Not only did I miss it, but I disagree with the answer that was
>> given.  What is your take?
>>
>> The World Wildlife Fund has declared that global warming, *a phenomenon
>> most scientists agree to be caused by human beings in burning fossil fuels*,will
>> create havoc among migratory birds by altering the environment in ways
>> harmful to their habitats. A. a phenomenon most scientists agree to be
>> caused by human beings in burning fossil fuels, B. a phenomenon most
>> scientists agree that is caused by fossil fuels burned by human beings, C.
>> a phenomenon that most scientists agree is caused by human beings' burning
>> of fossil fuels, D. which most scientists agree on as a phenomenon caused
>> by human beings who burn fossil fuels, E. which most scientists agree to be
>> a phenomenon caused by fossil fuels burned by human beings,
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