Hi abhasjha,

The correct answer is C. Let's take a quick look at all the answers to see what is wrong with the incorrect ones:

(A) As it is with traditional pharmacies, on-line drugstores rely on prescriptions to be successful

Incorrect. What does "it" refer to? We're not sure what is being compared.

(B) As with the case of traditional pharmacies, on-line drugstores rely on prescriptions to have success

Incorrect. Again, we're not sure what is being compared. What case, specifically?

(C) As is the case with traditional pharmacies, prescriptions are the cornerstone of a successful on line drugstore

Correct. Hooray!

(D) As traditional pharmacies, so on-line drugstores rely on prescriptions to be successful

Incorrect. "As traditional pharmacies..." is not correct sentence structure.

(E) Like traditional pharmacies, the cornerstone of a successful on-line drugstore is prescriptions

Incorrect. "Like traditional pharmacies..." is not correct sentence structure.