The correct option is C Beta particles
The force experienced by a moving charged particle in a magnetic field is given by = q v x B, where q is the magnitude of charge, v is velocity and B is external magnetic field. So, the particle experiencing maximum force would be deflected the most. Alpha particle has a charge of +2 and beta has a charge of -1. So, assuming same velocity and magnetic field, more would be the charge more would be the force hence deflection. So, alpha particle would be deflected the most as it has a charge of +2.