What's a "methodological approach" for research (Master's thesis)?
You can't really choose your topic definitively until you have "mastered" the related material, but you can give a strong, well-informed description of what your topic might be.

A methodological approach is the approach you will take to exploring your topic. For example: "I will perform neural network simulations of the PFC and BG of a decision making agent while recording neuronal activity and behavior. I will supplement this computational approach with validation of behavioral results in humans." The approach should be carefully thought out (as the above example was not) and worded to make it sound nontrivial yet feasible within the scope of your program.