Which would ultimately cause the armature in an electric motor to spin?

The armature spins because magnetic forces act on its current-carrying sides. As it turns through each half-rotation, the commutator reverses the current, which reverses the forces and keeps the armature turning in the same direction. So reversing the armature current ultimately causes it to spin; stopping the current or losing contact with the brushes would stop it.

AI-written and checked against the verified answer.