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An em wave is propagating in a medium with a velocity $\vec{V} = V\hat{i}$. The instantaneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along
Explanation
$\vec{E}\times\vec{B}$ points along the propagation direction.
Detailed Solution
$\vec{E}\times\vec{B} \parallel \vec{V}$
$(E\hat{j})\times(\vec{B}) \parallel V\hat{i}$
Since $\hat{j}\times\hat{k} = \hat{i}$, $\vec{B} = B\hat{k}$
So the magnetic field oscillates along the +z direction.