Final answer:
The statement is true; inserting a dielectric into an air-filled capacitor reduces the electric field because the dielectric material opposes the field created by the charges on the capacitor plates.
Step-by-step explanation:
The statement that the electric field in an air-filled capacitor is reduced when a dielectric is inserted between the plates is True. When a dielectric material is placed between the plates of a capacitor, the electric field strength within the capacitor is decreased. This reduction in electric field occurs because the dielectric material becomes polarized in the presence of the electric field, resulting in the creation of bound charges on the surface of the dielectric which oppose the field created by the free charges on the plates. The net effect is a decrease in the overall electric field strength within the capacitor.