Which force(s) cause a torque on the yoyo

I am doing a lab for Physics and this question stomped me. I know the answer is Tension, but how do I explain why?

Thanks for all the help. The most detailed question gets 10 points.

The torque is caused by the string tension wanting to resist the weight of the yo-yo. As the only time the vertical tug of the string works along the vertical line of the center of mass of the yo-yo is at the exact bottom of the drop, most of the time the string force also has what's called a moment arm. The moment arm, acting over time with the tension in the string, generates the torque and resultant spin.