I am lost, problem with a 555 timer

Hi.
See pic:
http://www.flickr.com/photos/67251822@N0…

What is the correct answer?

Everything is given in this question. Forget about the answers that you don't have information about, like lowering the supply voltage or changing the trigger.

T0 = 1.1 RC.
you want about 50mS, so first plug in C=2.2uF answer, and you get T0 = 1.1 x 100k x 2.2uF = 242mS
So that's not the answer.

If you use R=47K, then T0 = 1.1 x 47K x 1uF = 51.7mS. That's closest and that's the answer.