Question has been edited for clarity; Official Answer is:

Official Answer:

This sentence begins with a modifying phrase (a phrase which describes something and does not have a subject on it's own). When a sentence starts with such a phrase, the noun following the comma must be the thing which is described. Thus, in this sentence, the noun after the comma must be the thing which is sold over the counter and pharmacists are certainly not sold over the counter. Thus A and B are incorrect.

Because we have to fix this by fixing the begining of the sentence and none of the sentences describe pharmacists, we must fix the sentence by changing it from a modifying phrase to a clause - by giving it a subject and a verb. Each of C, D and E have pronoun sentences so we have to check refferring nouns. D describes selling "it", while C & E describe selling "them". Because cocaine is what is being sold, and cocaine is singular, it is the correct form and C& E must be eliminated. Leaving D as the correct answer.

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My take D

A) Often sold over the counter one hundred years ago---this needs to modify cocaine ,whereas modifying pharmacists

B & C has the same modifier error.

Between D and E

E is wrong in the usage of wrong tense form.

MORE TO ADD

E) Although they were selling them one hundred years ago===.THEM IS wrong AS COCAINE IS SINGULAR.
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Hi,
I have a question on your analysis, how does C describe "selling them", the sentence in C states that that as "it" and not them?

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