A plane electromagnetic wave propagating along y – direction can have the following pair of electric field  E→ and magnetic field  B→ components.

EM wave is propagating in y-direction. E→  and  B→ vectors are perpendicular to direction of propagation of wave. Hence neither of them have a component along y-axis. Both vectors have to be in x-z plane. Ex,Bz or Ez,Bx  is the correct answer