Final answer:
We cannot determine the exact velocity of the proton after the collision without more information about the mass of the helium atom and the type of collision.
Step-by-step explanation:
The velocity of a proton after it collides with a stationary helium atom can be determined by applying conservation of momentum, assuming no other forces are involved in the collision. However, without the mass of the helium atom or further information about the collision (elastic or inelastic), we cannot resolve the final velocity of the proton with the information provided. We would also need to know if the collision is head-on or at an angle. If this were an elastic collision in one dimension with a much heavier target like a helium atom, we could possibly expect the proton to bounce back with its original speed, but with an opposite direction (Option C would be correct in this specific and simplifying case).