This is an LSAT question not appropriate for the GMAT forum.

Generally speaking, on assumption questions one starts by identifying the conclusion of the argument. However, since this is an LSAT question, I will use LSAT terminology and call it the main point. The main point can be identified by many ways, but one way is to look for modal verbs such as will, should, and must. Our leading candidate is "Once that happens(the enzyme is restored to normal levels), we will be able to eliminate periodontitis."

Why? Well, the argument doesn't really say. It does say that lack of the enzyme reduces a person's ability to fend off periodontitis. It does not, however, say that the enzyme is sufficient all on its own to fend off periodontitis. It seems to assume that.

Answer choice (A) offers us the assumption that the enzyme is the only way to fend off periodontitis. Even if this is false and there are other ways to do so, that does not prove that restoring the enzyme to proper levels will not eliminate periodontitis.

Answer choice (B) offers us the assumption that only the genetic mutation caused reduced enzyme. This is irrelevant. The plan is simply to restore the enzyme to normal levels regardless of whether a genetic mutation was the cause.

Answer choice (C) suggests that researchers will soon reach this breakthrough. This is irrelevant. Even if researchers take a century to do so, it does not mean that once it has been accomplished that periodontitis will not be eliminated.

Answer choice (D) suggests that people without the mutation do not get gum disease. Even if this is false, it does not mean that restoring the enzyme to normal levels will not work. The cause of the reduced enzyme (genetic or otherwise) is irrelevant to the argument.

Answer choice (E), the credited response, is the assumption we need. If we negate this choice it reads: "A person whose cathepsin C level has been restored to normal will suffer from periodontitis." If that is true, the the plan is doomed to failure. Accordingly, we must assume that this is not true for the argument to make sense.