Let's deconstruct the sentence:
As it is with traditional pharmacies-
The author here is trying to compare something with "it is" of the traditional pharmacies on-line drugstores rely on prescriptions to be successful,
OK so we are comparing dependence of success of both on-line and traditional pharmacies on prescriptions. Cool!since it is primarily prescriptions that attract the customers, who then also buy other health-related items.
Reason behind why prescriptions are important.So, the author wants to say that- for an online pharmacy to be successful, it has to rely a lot on prescriptions. And this dependence on prescriptions for success is no different in the case of traditional pharmacies.
Let's look at the options:
a) As
it (what does "it" refer to? It is not a placeholder either, but we cannot elimate just because of it, lets look for more errors )is with traditional pharmacies, on-line drugstores rely on
prescriptions to be successful This is nonsensical. The Author intends to say that for the store to be successful, prescriptions are important. prescriptions to be successful does not make sense.
b)
As with(there is some problem with "as with" I am not sure what it is

) the case of traditional pharmacies, on-line drugstores rely on prescriptions
to have success "To have success" is awkward
c) As is the case with traditional pharmacies, prescriptions are the conrnerstone of a successful on-line drugstore- good.
The intended meaning and comparison with the traditional pharmacy is aptly done.d)
As traditional pharmacies(as traditional pharmacies what? As followed by noun acts as a function, we need a comparison here), so on-line drugstores rely on
prescriptions to be successful (as discussed above, prescriptions to be successful is distorting the intended meaning.
e)
Like traditional pharmacies(we are not comparing traditional pharmacies, but we are comparing the dependence on prescriptions), the cornerstone of a successful on-line drugstore is prescriptions