A plane electromagnetic wave propagating along x direction can have the following pairs of E and B
We know that in an electromagnetic wave the electric and magnetic field are perpendicular to each other and both are perpendicular to the direction of propagation of the wave. Therefore, for a wave propagating along x direction the electric and magnetic field can be along y an z axes. Therefore, the possible pairs of E and B would be, Ez, By and Ey, Bz.