Assumptions are unsaid premises. They are those missing links that would complete the story. Everything moves fine till we encounter lasts sentence. That seems a bit out of place. Stem says that shy kids develop hobbies---> hobbies help them find friends ---> is they loose hobby then loneliness increases (fine) ---> Therefore, hobby is not a good idea?? Well there is a missing link between the last two lines.

And see that B is the only option that fits there. Hobby decreases loneliness---> loss of hobbies decrease loneliness---> any strategy that has a risk of intensifying loneliness isn't a good strategy---> Therefore, hobby isn't a good strategy.

i was nonplussed between B and D. But why not D ?

The author concludes that "developing an all-consuming hobby is not a successful strategy for overcoming adolescent loneliness." How does the author arrive at that conclusion?

If an adolescent is shy, his/her shyness can cause loneliness.
To distract themselves from the loneliness, shy adolescents often immerse themselves in an all-consuming hobby.
Sometimes, those adolescents meet others who share the same hobby and develop friendships with those other people.
For example, two shy adolescents might become friends because they both enjoy playing the same video game.

But what if those adolescents lose interest in the hobby?

The friendship between the two shy adolescents, for example, was based on enjoying the same video game.
If one person no longer enjoys that video game, the friendship will likely fall apart.
Now the shy adolescents will feel lonely again... maybe even more so than before because they had a friend and lost him/her.
In other words, the loneliness can be exacerbated, or intensified.

So developing an all-consuming hobby can certainly intensify the adolescents' loneliness. Because of this, the author concludes that developing an all-consuming hobby is not a successful strategy for overcoming adolescent loneliness.

But does the fact that the strategy might intensify the adolescent's loneliness mean that the strategy will not help the adolescent overcome the loneliness?

What if the intense loneliness actually helps the adolescent overcome the loneliness?
For example, maybe the intensified loneliness was so bad that it motivates the adolescent to face his/her shyness and try to talk to more people.
In that case, a strategy that intensified the loneliness could also help overcome the loneliness.
This example would expose a flaw in the argument.
Choice (B) eliminates this possibility, so it is a required assumption.

Choice (C) talks about what happens if the adolescents do NOT make friends through that hobby.

Perhaps the adolescents won't lose interest if they don't make any friends through that hobby.
In that case, they would remain distracted from their loneliness, but they will not overcome their loneliness.
More importantly, the argument focuses on what happens when they DO make friends through the hobby.
The hypothetical situation described in choice (C) does not NEED to be true for the argument to hold. Eliminate (C).

(B) is the best answer.