the magnitude of $B$ is
$B=\large\frac{E}{c}$$ = \large\frac{6.3}{3 \times 10^8}$$ = 2.1 \times 10^{-8} T$
To find the direction, we note that $E$ is along $y$ - direction and the wave propagates along $x$ - axis.
Therefore, $B$ should be in a direction perpendicular to both $x$ - and $y$ - axes.
Using vector algebra,$ E\times B$ should be along $x$ - direction.
Since, $(+j) \times (+k) = i$ , so, $B$ is along +k or the $z$ - direction.
Ans : (c)