Light wave is travelling along y-direction. If the corresponding E vector at any time is along the x-axis, the direction of B vector at that time is along

-z-axis

The given wave in an Electromagnetic Waves. Electromagnetic radiation is a self propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation.

Direction of propagation is along $ \vec{E}\times \vec{B}$

It is given that Direction of propagation is along $\hat{j}$ , Direction of E is along $\hat{i}$ hence direction of B must be along $\hat{-k}$