A sociologist recently studied two sets of teenagers. The members of

A sociologist recently studied two sets of teenagers. The members of one set
spent 10 or more hours per week watching violent television programs, and the
members of the other set spent 2 hours or less per week watching violent television programs. A significantly greater proportion of the teenagers in the
former group exhibited aggressive behavior during the period of the study. The
sociologists reasoned that the prolonged exposure to television violence caused
the aggressive behavior.
Which of the following, if true, of the teenagers in the study, provides the
strongest challenge to the sociologist’s conclusion?
(A) Some teenagers who watched more than 10 hours of violent television
programming per week behaved less aggressively than others in the same
group of teenagers.
Incorrect - as this is already given in the passage.. Not new and so no effect on conclusion

(B) Some teenagers who watched 2 hours of violent television programming per
week did not behave aggressively.
Incorrect - as this is already given in the passage.. Not new and so no effect on conclusion

(C) Some teenagers voluntarily stopped watching violent television programs
after being victims of violence.
Incorrect - this statement makes the stem stronger as the cause - effect relation is confirmed.

(D) Some teenagers watched violent television programs alone, while others did
so in groups.
Irrelevant - Shows no other cause in place

(E) Many of the teenagers in the first group exhibited aggressive behavior before
the study began.
Correct - as it gives the reason that teenagers were already aggressive

Another way of looking towards is that the stem talks about a significant proportion of the teenagers. Hence Some teenagers (present in A, B , C, D options) makes a little impact to the stem...!

Having said that... I am myself a little confused... as E says that Many of the teenagers in the first group were already aggressive before the start of the study... but I ask this question to myself.... How about they were watching the violent programs before the start of the study itself.... No where in the stem it is mentioned that teenager groups or set started to watch violent programs at the start of the study!.... Kinda lost.. . but of course... E seems to be the best choice...