This is an Evaluate question, meaning we need to select the answer choice that will help us to determine whether the argument is valid.
The author wants to prove that patients should take the new drug along with the drug that helps to preserve existing bone. Why? Because the drug enhances growth of new bone cells, and these patients are treated with drugs that result in bone loss (osteoporosis).
This is a classic pattern on the LSAT. It is very common for LSAT arguments to conclude that we SHOULD do something from the fact that it seems to have certain benefits. But this is problematic for a few reasons:
How do we know there are no disadvantages that outweigh the benefit of bone cell growth? For example, what if this new drug is actually bad for your health overall?
How do we know this supposed benefit (of new bone cells) is even helpful? The patients are already being treated with drugs that help preserve the bone. What if that is all they need, and taking this new drug along with it is only going to cause problems?
We want an answer that will address either of these two concerns.
(A) This answer is irrelevant. The size of the class of drugs has no bearing on whether the drugs will actually be helpful, let alone whether they should take these drugs
(B) This answer is irrelevant. The reason why the patients are treated with the drugs does not help us determine whether the new drug is beneficial along with the drug that helps with bone preservation. You might be thinking that this answer questions whether we need these patients to take the drugs that cause the problem in the first place. But if anything, this is questioning the premise of the argument, which we cannot do. We can only question whether the conclusion follows from the premise.
(C) This initially may seem to be a decent answer. After all, if the drug is prohibitively expensive, maybe that is a disadvantage, and people should not use it. However, that is not what C is saying. Just because one drug is MORE expensive than another does not mean it is expensive, because we don't know how much the other drug is to begin with.
(D) This answer is irrelevant. It may be tempting to think that how long it has been used can help us determine whether it is safe to use, but we don't know that for sure. After all, there are plenty of things that are dangerous that have been around for a very long time. Also, notice how this answer is actually talking about the bone preservation drug, not the one that the conclusion is proposing.
(E) This is our answer. If the drug does not remain effective when used in combination with other drugs (aka the bone preservation drug), there would be no benefit of taking the drug proposed by the conclusion. This falls under the second bullet point predicted above.
Therefore, answer choice E is correct.