Question 1
What does the difficulty of stopping a moving object depend on?
The right answer is the one marked above.
Both mass and velocity. Momentum is the measure of that difficulty.
A shot put is hard to catch when it is thrown, but not when it is handed to you — same mass, different velocity. A bullet has a tiny mass, yet once fired from a gun, stopping it is not worth thinking about.
So looking at either one alone is not enough. Only when both are large does it become dangerous.