When the stem says weaken EXCEPT all the four incorrect options must weaken the conclusion. the correct answer must either strengthen or not have any effect at all on the conclusion.

What does the argument state ?
Prozac has fewer side effects --> Revenues from selling prozac will far exceed those from selling Elavil.
Note the word "far exceed".

A - Weakens the argument. Elavil is used as an anti-emetic in addition to its use as an anti-depressant. It has an additional source of revenue that prozac does not have. hence, it is unlikely that revenues from prozac will far exceed revenues from elavil.
B - Weakens the argument. If prozac cannot be sold in six states, then it is unlikely that revenues from prozac will far exceed revenues from elavil.
C- Correct answer. The number of side effects they have in common does not affect my argument as long as prozac has fewer side effects than elavil has. Also, note the word "some". Some can mean just 2 side-effects or 100 side-effects (a lot of side effects in common). If it is just 2 in common, this option does not weaken the argument. Even if the number is high, what if the few additional side effects that elazil has are far more serious? Then certainly, my argument would not be weakened.
D- weakens the argument. It cannot be targeted to diabetics and heart patients.
E - If price of prozac is lower, then the demand for the product will be higher. However, note that revenues = price * demand. So, the revenues might not be so high after all. Look at the argument again. It says - "revenues will far exceed". This does not seem likely since the price of prozac is lower. Hence, weakens.