Which word signals a nonrestrictive clause in a complex sentence?

-that

-which

-while

-yet

First of all, you need to know what a nonrestrictive clause is. It is a type of a clause which is dependent, but also not really needed in a sentence for it to have a proper meaning. For example: Spain, which is a humid country, is located in Europe. Nonrestrictive clauses are often set off by commas.

Having all of this in mind, the correct answer is which.