okay. I am putting my explanation to those who didn't choose option C as answer.

Premise: For sinus pain, 3 out of 4 hospitals give Novex. that is 75 % hospital use medicine Novex as pain killer.
Conclusion: Novex is the most effective pain killer.

We need to find something that will weaken conclusion 'Novex is the most effective'. Without looking at the option, if you would question, why Novex is not most effective even 75% hospitals use this medicine ?
There could be below possibilities.
1. Novex is used everywhere because, it is abundantly available.
2. The most effective pain killer may not be affordable to most of the patients.

Now going through the options
(A) Some competing brands of painkillers are intended to reduce other kinds of pain in addition to sinus pain. Argument talks only about sinus pain.
(B) Many hospitals that do not usually use Novex will do so for those patients who cannot tolerate the drug the hospitals usually use. Argument talks about effectiveness and not tolerance.
(C) Many drug manufacturers increase sales of their products to hospitals by selling these products to the hospitals at the lowest price the manufacturers can afford. Yes. Most of hospitals use Novex because it is easily affordable to everyone. It may not be the most effective.
(D) Unlike some competing brands of painkillers, Novex is available from pharmacies without a doctor’s prescription. No one will use any medicine that is sold without prescription knowing that it is not effective.
(E) In clinical trials Novex has been found more effective than competing brands of painkillers that have been on the market longer than Novex. It strengthens the argument.

(A) Some competing brands of painkillers are intended to reduce other kinds of pain in addition to sinus pain.--> Out of scope
(B) Many hospitals that do not usually use Novex will do so for those patients who cannot tolerate the drug the hospitals usually use--> Not useful as per the passage
(C) Many drug manufacturers increase sales of their products to hospitals by selling these products to the hospitals at the lowest price the manufacturers can afford----> Gives an alternate explanation as to why hospitals refer Novex to its patients. Weakens the argument.
(D) Unlike some competing brands of painkillers, Novex is available from pharmacies without a doctor’s prescription---> Gives the reason for the availability of Novex at pharmacies not hospital
(E) In clinical trials Novex has been found more effective than competing brands of painkillers that have been on the market longer than Novex----Strengthening the argument.