Yes, B is the answer to this question.
I'm confused am i supposed to answer or read
The correct answer is that it is a dependent clause.
A dependent clause, as the name itself says, depends on a larger clause in order to make sense. As you can see in the example above, that clause is incomplete - it needs to be part of a larger, independent clause in order to get a proper meaning. The other options are all complete sentences, which is something the example is definitely not.
C. By limiting the amount of coverage any one topic gets
