If a plane electromagnetic wave satisfies the equation∂2Ex

If a plane electromagnetic wave satisfies the equation
∂²E_x = e² ∂²E_x
∂z² ∂t²

the wave propagates in—

1. x-direction
2. z-direction
3. y-direction
4. xy plane at an angle of 45º between x and z direction

For the propagation of plane electromagnetic wave their is change in the intensity (magnitude) of wave in the direction of propagation with the time.

∂²E_x	= e²		∂²E_x
∂z²			∂t²