Final answer:
Singly ionized helium ions will curve the most in a magnetic field when compared to iron and uranium, due to helium's smaller mass and higher charge-to-mass ratio.
Step-by-step explanation:
The curvature of an ion in a magnetic field depends on its charge-to-mass ratio. When ions have the same speed and charge, the lighter one will curve more in a magnetic field because it has a higher charge-to-mass ratio. Among iron, uranium, and helium, singly ionized helium ions will curve the most since helium has the smallest mass compared to the singly ionized ions of iron and uranium. Therefore, the answer is helium.