Hi
Please find the explanation below:
According to one expert,
the cause of genetic irregularities in many breeds of dog is not so much that dogs are being bred for looks or to meet other narrow criteria as that the breeds have relatively few founding members.
A. the cause of genetic irregularities in many breeds of dog is not so much that dogs are being bred for looks or to meet other narrow criteria
--> This has the correct idiom usage and structure - the cause of genetic ... is not so much X .... as Y. Here both X and Y should have a parallel structure. We can notice that the Y portion has the structure that begins with 'that the breeds ...' so the X portion should also begin with 'that dogs are ....' This alone can be used to rule out the other answer choices.B. the cause of genetic irregularities in many breeds of dog is not as much their being bred for looks or meeting other narrow criteria as much
--> the structures are not parallel i.e the X portion begins with 'their being bred ...' which is incorrect as it should begin with 'that ... '.C. it is not so much the cause of genetic irregularities in many breeds of dog that they are being bred for looks or meeting other narrow criteria as much
--> structure not parallel and the use of the pronoun 'it' in the begin makes the sentence confusing. What is 'it' referring to?D. it is not so much the cause of genetic irregularities in many breeds of dog is their being bred for looks or meeting other narrow criteria so much
--> structure not parallel and the use of the pronoun 'it' in the begin makes the sentence confusing. What is 'it' referring to?E. it is not so much the cause of genetic irregularities in many breeds of dog to be bred for looks or to meet other narrow criteria
--> structure not parallel and the use of the pronoun 'it' in the begin makes the sentence confusing. What is 'it' referring to?Hence the correct answer is
option A.
Hope this helps.

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