I'm wondering if this sentence
Optical fibres, which are made from very pure silica fibre, are the form of transmission line which is most often used these days.
that the relative clause is which is most often used these days (if I'm not wrong).
So I have three guess to reduce:
1.- most often used these days
2.- being most often used these days
3.- no possibility to reduce
What is the correct guess? And why?