An electron moving with uniform velocity in x direction enters a region of uniform magnetic field along y direction. Which of the followed physical quantity(ies) is (are) non-zero and remain constant? 

1. Velocity of the electron.
2. Magnitude of the momentum of the election.
3. Force on the electron.  
4. The kinetic energy of electron.  

All four

When an electron moves in the external magnetic field, a magnetic force acts on the electron perpendicular to the direction of motion of the electron. Thus, magnetic force does not do any work on the electron. Hence, its kinetic energy remains constant. Therefore, momentum and velocity are also constant. 

Since, magnetic field and velocity are constant, Lorentz force is also constant.