Not necessary, because the author never argued that someone might lose interest in the hobby because of a desire for more friends. The author makes no comment on what might lead to a loss of interest in the hobby.

Necessary, because if it were not true — if SOME successful strategies for overcoming adolescent loneliness CAN intensify that loneliness — then the mere fact that an all-consuming hobby might exacerbate loneliness would not prove that it’s not a successful way to overcome adolescent loneliness.

Not necessary, because the author never argued that someone might lose interest in the hobby because they don’t make enough friends. The author makes no comment on what might lead to a loss of interest in the hobby. You can also analyze (C) using negation. Even if shy adolescents won’t lose interest from not making enough friends, that still leaves open the possibility that they could lose interest for other reasons. And in that case, loneliness can still get worse.

Not necessary, because the author doesn’t have to believe there’s a successful strategy for overcoming adolescent loneliness. All that the author argues is that one particular strategy isn’t successful; this doesn’t imply there are other strategies that are more successful.

Not necessary, because the desire to make friends does not have to be the “main” reason shy adolescents devote themselves to hobbies. If it’s only the second most important reason, that doesn’t impact the author’s argument. In fact, it doesn’t have to be part of the purpose of getting into a hobby at all. The argument relies on the fact that losing interesting in a hobby can make loneliness worse — whatever the original reason for getting into the hobby, a loss of interest can make loneliness worse.