the correct answer is C
but how about " of what" ? for example, The earth is 49 times the size of the moon.

miachi wrote:A new hair-growing drug is being sold for three times the price, per milligram, as the drug's maker charges for another product with the same active ingredient.

A. as
B. than
C. that
D. of what
E. at which

IMO C is the best choice here. That introduces the necessary clause and forms the correct sentence structure. Also note the comma splice here

miachi wrote:A new hair-growing drug is being sold for three times the price, per milligram, as the drug's maker charges for another product with the same active ingredient.

A. as
B. than
C. that
D. of what
E. at which

Example :

your machine costs three times that of my machine.

your drug is sold for three times that of my other products. (here "the price per milligram" is the extra information that does not harm the logic of the sentence.)

The only thing I am bothered about is that there is no OF after THAT in the OA.

According to me the sentence should have been :

A new hair-growing drug is being sold for three times the price, per milligram, that of the drug's maker charges for another product with the same active ingredient.

Only "that" cannot make logic or compare the products in the sentence. It should have been "that of"

Now, "of what" is totally correct that I feel although "that of" is the best.
But since we don't have "that of'. I think "of what" seems good.

Example :

Your store charges three times of what my store charges for this machine.

Government's revenue is three times of what State revenue is.

Though "Government's revenue is three times that of the State revenue" is the best

What is the source of this question?

If the options are correct. I will go with "of what"

A new hair-growing drug is being sold for three times the price, per milligram, as the drug's maker charges for another product with the same active ingredient.

A. as
B. than
C. that
D. of what
E. at which

Grammar:

Relative Pronoun and Antecedent:

The noun or pronoun immediately before relative pronoun is called antecedent.
Example:
I know they boy who is a doctor.
=> The boy is the antecedent of the relative pronoun who.

Relative pronoun and antecedent should stand side by side.
So, Antecedent +Relative Pronoun=A+RP
Example of relative pronouns:
THAT, WHICH, WHO, WHAT.

Few words about WHAT:

What is the special kind of relative pronoun that works both as a relative pronoun and antecedent. It means that WHAT has two powers.

i.e., WHAT = A+RP.

Example:
I said some thing. This is true.
If we join these two by which/that, it looks:
I said something that is true. [A+RP =Something+that]

If we join by WHAT:
I said what is true. [Something +that =A+RP =WHAT]

Now, come to the sentence:

It is not comparison. So, the use of "as" and "than" is not expected. There is an antecedent "the price". So, the use of what is not required. A+RP will be used directly. There will be no middle man between A and RP. So, "at which" is not expected. Which one remains? It is C, which is the correct option.

IMO is C

Nice explaination Hrishi

Why is B wrong? When you compare two things, don't you use "than" to show the comparison?

Thank you!