I believe that the OA for this question is incorrect. It should be C. Here is why:

Though certain breeds of dog are renowned for their sense of smell, there is often a greater difference in scenting ability between two members of a single breed as there is between members of different breeds.
What does the sentence mean? This sentence states a comparison. The comparison is between the following entities
1: Difference in scenting ability between 2 members of a single breed
2: Difference in scenting ability between 2 members of different breeds
The sentence states that 1 is greater than 2

What are the errors in the original sentence? To determine the errors we should first understand the sentence structure:
1: Though certain breeds of dog are renowned for their sense of smell,
2: there is often a greater difference in scenting ability between two members of a single breed as there is between members of different breeds.

Clause 2 has inverted structure. In order to identify the correct SV, we need to write this sentence is normal structure:
The difference in scenting ability between 2 members of a single breed is often greater than the difference in scenting ability between members of different breeds.
The SV pair has been identified as shown above. It is important to note that "there" is not a subject here.
The original sentence has only 1 error = idiom greater..as is incorrect. It should be greater..than.

Process of Elimination: Based on idiom error, we can eliminate choices A, B, and D. This leaves Choices C and E.

(A) is often a greater difference in scenting ability between two members of a single breed as there is
(B) is often more difference in scenting ability between two members of a single breed as to find one
(D) are often greater differences in scenting ability between two members of a single breed as there are
(C) is often a greater difference in scenting ability between two members of a single breed than
Correct answer.
(E) are often greater differences in scenting ability between two members of a single breed than there is
Incorrect because it sets incorrect comparison as shown (as explained by crackverbal as well)
differences in x between "abc" are greater than it is between "xyz"

I hope this helps.

Regards,

Payal