As light propagates through a vacuum, an electric field and a...

Light is an electromagnetic wave. In an electromagnetic wave propagating through a vacuum, the electric field $\vec{E}$ and magnetic field $\vec{B}$ are mutually perpendicular to each other and both are perpendicular to the direction of propagation of the wave.

The variations of the electric and magnetic fields are given by $\vec{E} = \vec{E}_0 \sin(\omega t - kx)$ and $\vec{B} = \vec{B}_0 \sin(\omega t - kx)$.

Since the fields vary sinusoidally with time, they do not remain constant.

The average value of a sinusoidal function over a complete cycle is zero. Therefore, the average values of the electric and magnetic fields are zero.