Answer:
For many young people sport is a popular part of school life and being in one of the school teams is very important.
Explanation:
when you put the other options in the blank, it simply doesnt make much sense :)
"being in" means taking part in
[if u r confused, no you cant put "taking in" because it means 'absorbing/understanding/accepting'. E.g. I am taking in the scenery]