Answer
(a) increase
(b) I
Work Step by Step
(a) We know that $r=\frac{mv}{qB}$. This shows that the radius is directly proportional to the the speed of the particle, thus if the speed of the particle is increased then the radius of each half segment will be increased.
(b) We know that the best explanation is option (I) -- that is, the radius of a circular orbit in a magnetic field is proportional to the speed of the proton, therefore, the radius of the half-circular path will increase.
