The striked out part is just the fluff in the sentence now we need to understand .
Meaning : The sentence talks about the Harris Tweed which was once made from wool of sheep of coast of Scotland , but are not imported sometimes from mainland or sometimes from outside.
We need to maintain parallelism here as soon as we see this question i.e. "Sometimes from the mainland and sometimes they come " is toally incorrect this eliminates A , D and E .
Leaves you with B and C .
In C we have that ... which sounds incorrect ?
So leaves us with B as the answer .