Critical reasoning problems are best tackled by reading the question first. In it we see that there is an argument with a conclusion and that we are looking to identify the assumption.

How can we find the conclusion? There is no one bulletproof answer, but conclusions generally share a few features:

1. Indicator words such as therefore, thus, hence, clearly, etc.
2. Modal verbs such as must, should, will, would, etc.
3. Express opinions/claims rather than facts.
4. Rely on other parts of the argument to make sense.

In this case, our top candidate for the conclusion is the sentence: Federal legislation that prohibits the sale of video games to minors would help curb this painful wrist condition among adolescents.

It contains the modal would and expresses the doctor's opinion.

Are there any new, surprising words in that conclusion - words not mentioned elsewhere in rest of the argument? I think there are. The first sentence talks about video games and carpel tunnel syndrome whereas the conclusion talks about federal legislation.

In our own words, we can predict that the assumption is something like "Federal legislation prohibiting the sale of video games to minors will prevent minors from playing video games and thus prevent carpel tunnel syndrome."

Now we go to the answer choices. While doing so, we will state the conclusion, ask the question "Why?" and see whether the answer choice answers that question.

Federal legislation that prohibits the sale of video games to minors would help curb this painful wrist condition among adolescents.

Why? Because (A) The majority of federal legislators would vote for a bill that prohibits the sale of video games to minors.

Does that make sense? Not really. Even if this were false, it wouldn't affect the doctor's conclusion that federal legislation would help.

What about (B)? Does that answer the question why? Not at all.

What about (C)? It doesn't answer the question why and it doesn't really strike at the heart of the argument. The doctor only says that federal legislation would help, so talking about other ways an adolescent could develop carpal tunnel syndrome is irrelevant.

Answer choice (D), on the other hand, makes sense. However, to be certain that this is the correct answer, we will negate option D. If it really is the best answer, then the negated version should weaken or destroy the conclusion.

Federal legislation that prohibits the sale of video games to minors would help curb this painful wrist condition among adolescents.
(D) (negated version) Most parents would simply purchase video games for their children.

Here we can see that the conclusion is in serious doubt. Prohibiting sales to minors would be completely ineffective if the parents are willing to purchase video games for their children.

Accordingly, we can see that (D) is the best answer.