Question

Identify the subordinate clause. The book i am reading concerns world population problems

Answer 1

Im not completely sure but i think the subordinate clause is "concerns the world population"

Answer 2

A subordinate clause is a dependent clause that cannot stand on its own.
The subordinate clause here is I am reading. 
The whole sentence is The book (that) I am reading concerns world population problems.
As you can see, the independent sentence is The book concerns world population problems, because it can stand on its own.
That I am reading cannot.