Which component rotates past the field coils to create opposing magnetic fields?

The armature is the correct choice because it plays a crucial role in electrical machines, such as generators and motors. The armature is typically made up of windings of wire and is positioned within the magnetic field created by the field coils. When the armature rotates, it cuts through the magnetic lines of force produced by the field coils, leading to the generation of electromotive force (EMF) through electromagnetic induction.

This interaction between the rotating armature and the stationary field coils leads to the creation of opposing magnetic fields, which is integral to the machine's operation. The direction and strength of these induced currents will depend on the speed and orientation of the armature rotation relative to the magnetic field.

The other components, such as brushes, starter housing, and pinion gear, do not have a direct role in creating opposing magnetic fields. Brushes are crucial for conducting current to the armature while it rotates. The starter housing provides structural support, and the pinion gear engages with other mechanical components but does not directly interact with the magnetic fields in the same way the armature does.