Final answer:
A singly ionized helium atom will curve the most in a magnetic field when compared to iron and uranium atoms of the same speed, because helium has the lightest mass.
Step-by-step explanation:
The magnitude of the force experienced by a charged particle moving in a magnetic field is given by the Lorentz force equation, F = q(v x B), where F is the force, q is the charge of the particle, v is the velocity, and B is the magnetic field. For particles with the same charge and velocity, the force is the same, but the effect of the force depends on the mass of the particle since F = ma (Newton's second law of motion). The particle with the least mass will accelerate the most and therefore curve the most in the magnetic field. Among the given options, helium (He) is the lightest element, and thus, a singly ionized helium atom will curve the most.