The answer is: Because George does not understand
A subordinate or dependent clause cannot stand alone because it does not express a complete thought. It begins with a relative pronoun or a subordinate conjunction (like because), and contains a subject and a verb.
Subordinate clauses need to be connected to an independent clause in order to make sense. For instance, a complete version of the example subordinate clause followed by an independent clause is:
Because George does not understand, he cannot solve the Maths problem.