How can I determine the quantum number of a electron lost during ion formation?
The correct answer has been given in the comments section of this post. I am providing the answer here in accordance with a meta post discussion.

The question you were asked was in reference to the quantum numbers that correspond to the electron lost when $\ce{Rb}$ goes to $\ce{Rb+}$. The electronic configuration of $\ce{Rb}$ is $[\ce{Kr}]\ 5\mathrm{s}^{1}$; thus, you need the set of quantum numbers for a $5\mathrm{s}^{1}$ electron. That set of quantum numbers is: 

$$n = 5;\ l= 0;\ m_{l} = 0;\ m_{s} = +{1\over2}$$