Two protons are moving with the same velocity in a particle accelerator.

Observer X is at rest relative to the accelerator. Observer Y is at rest relative to the protons.

Explain the nature of the force between the protons as observed by observer X and observer Y.

Y measures electrostatic repulsion only
protons are moving relative to X «but not Y» OR protons are stationary relative to Y
moving protons create magnetic fields around them according to X
X also measures an attractive magnetic force OR relativistic/Lorentz effects also present