Doctor: Research shows that adolescents who play video games on a regular basis are three times as likely to develop carpal tunnel syndrome as are adolescents who do not play video games. - fact
Federal legislation that prohibits the sale of video games to minors would help curb this painful wrist condition among adolescents. - claim by the doctor

Doctor is assuming that if Federal legislation prohibits the sale of video games to minors then the adolescent wont be able to avail it from any other source - This is the assumption in defender role, which helps the argument to stand from external attack.
This exactly is what "D" is.

Apart from this lets try negation technique to solve it :
A. The majority of federal legislators would not vote for a bill that prohibits the sale of video games to minors. - its not affecting the argument in any way ..
B.Not all adolescents who play video games on a regular basis suffer from carpal tunnel syndrome. - Already stated in the passage, and this is helping rather than weakening the argument in any way.
C. Playing video games is the only way an adolescent can develop carpal tunnel syndrome.- carpal tunnel syndrome is only the scope of the argument
D. Most parents would not refuse to purchase video games for their adolescent children.- This is a straight weakener and hence the right answer.
E. The regular playing of video games by adolescents does not produce such beneficial effects as better hand-eye coordination and improved reaction time. - A cause can have many effects, some benefitial. But the argument is mainly dealing with the carpal tunnel syndrome, so this wont stand.

IMO: D
I am sure that we finally penned down answer C and D.
Now D is better than C for following reasons.
1. Nowhere in the passage it is mentioned that playing video games is the cause of carpal tunnel syndrome. The passage just mentions that playing video games has more probability of carpal tunnel syndrome in adolescents.
2. Conclusion states : F.L prohibits the sale of video games to minors ,but what if Adult/minors parents buy the video games for their kids. -- This can weaken the conclusion to curb carpal tunnel syndrome in adolescents.

Yes D should be the answer.

But if we negate A then we get that the federal legislators will not vote for the bill and hence the bill will not be passed. Whether it will not affect the conclusion?