Light waves propagate in a vacuum along the X-axis. Which of the...

For a wave propagating along the X-axis, the direction of propagation is parallel to the X-axis.

Wavefronts are defined as the locus of all points having the same phase of oscillation.

For a plane wave traveling in the X-direction, the wavefronts are planes perpendicular to the direction of propagation.

A plane perpendicular to the X-axis is parallel to the Y-Z plane and has the equation $x = \text{constant}$.

Therefore, the equation $x = c$ represents the wavefronts of the light waves.