I believe it’s “C”
Hope that’s help you(:
Answer:

Explanation:
given,
width of door dimension = 1 m
mass of door = 15 Kg
mass of bullet = 10 g = 0.001 Kg
speed of bullet = 400 m/s


a) from conservation of angular momentum





Explanation:
It is given that,
Magnetic field, B = 0.15 T
Charge on a proton, 
Mass of a proton, 
The cyclotron frequency is given by :


f = 2286785.40 Hz
or


Hence, this is the required solution.
It'd be an unbalanced force