Imo D.
The concerns about feeding the growing global population has usually emphasized the possibility of available farmland and soil nutrients decreasing and to what extent it might, but agriculturists are more concerned about water scarcity caused by inefficient and uncoordinated water use that has reduced river flows, depleted aquifers, and caused salinization of irrigated land.
Meaning: The concerns about feeding of growing global population HAVE usually emphasized the possibility of X & Y and to what extent IT MIGHT, but agriculturists are more concerned about water scarcity caused by A & B that has impacts: P, Q & M.
SVA Error in A & E, The concerns, plural form, takes singular verb, has. Hence incorrect. Here, elliptic construction, without 'it might', makes sense to convey exact meaning.
A. The concerns about feeding the growing global population has usually emphasized the possibility of available farmland and soil nutrients decreasing and to what extent it might, - incorrect due to SVA error & wordiness
B. The concerns about feeding the growing global population have usually emphasized whether available farmland and soil nutrients would decrease and the extent that is, - SVA error corrected but another error, pronoun error, introduced, which led to meaning error.
C. Concerns about feeding the growing global population have usually had as its emphasis whether available farmland and soil nutrients would decrease and what that extent would be,- have usually has as its emphasis & usage of would on both side of 'and' change the meaning of the sentence.
D. Concerns about feeding the growing global population have usually emphasized whether available farmland and soil nutrients would decrease and to what extent, - SVA error is corrected & elliptical structure conveys meaning clearly. Correct.
E. Concerns about feeding the growing global population has usually had as an emphasis the possibility of available farmland and soil nutrients decreasing and to what extent it might, - incorrect due to SVA error & wordiness
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