Which sentence contains a nonrestrictive clause? be sure to watch that movie i gave you. the man who drives our bus is ill. take a left after that house with the yellow fence. the old apron, which belonged to mama, is torn.

It is relative easy to spot a nonrestrictive clause - it is usually set off from the rest of the sentence by commas. Nonrestrictive means that it is not essential for the sentence, which also means that if it were omitted from the sentence, the sentence would still make perfect sense. 
Having this in mind, the correct answer is - The old apron, which belonged to Mama, is torn.