Which clause is subordinate?

A.

Dad's jokes are always about math.

B.

Who talks about numbers all the time.

C.

Sometimes he recites lists of prime numbers.

D.

My father is a math teacher.

Answer: B. Who talks about numbers all the time.

Subordinate clause does not express a complete thought, therefore needs the independent clause to function properly. This subordinate clause is a noun clause. The sentence above is in imperative form though lacks question mark.