The correct option is A Decreases by 2%
We know the radius of a circular path of charge q moving with kinetic energy K in a uniform magnetic field is given by,
r=√2mKqB
As m, q and B are the same for the same charge.
∴r ∝√K
r=CK12 (where, C is a constant)
Taking log on both sides, we get
logr=logC+12logK
Differentiating both sides;
1r×dr=0+12(1K)dK
drr×100=12×dKK×100
As given, dKK×100=4%
∴drr×100=12×4=2%
Thus, radius will be decreasing by 2%.
Hence, option (a) is the correct answer.