A plane electromagnetic wave travels in free space along X - direction. If the value of $\vec{B}$ (in tesla) at a particular point in space and time is $1.2 \times 10^{-8} \hat{k}$, the value of $\vec{E}$ (in Vm^-1) at that point is:

$3.6 \hat{j}$

Here, $\vec{B}=1.2 \times 10^{-8} \hat{k} T$

The magnitude of $\vec{E}$ is

$E=Bc=\left(1.2 \times 10^{-8} T\right)\left(3 \times 10^8 ms^{-1}\right)=3.6 ~Vm^{-1}$

$\vec{B}$ is along Z - direction and the wave propagates along X - direction. Therefore $\vec{E}$ should be in a direction perpendicular to both X and Z axes. Using vector algebra $\vec{E} \times \vec{B}$ should be along X - direction.

Since $(+\hat{j}) \times(+\hat{k})=\hat{i}, \vec{E}$ is along the Y - direction.

Thus, $\vec{E}=3.6 \hat{j} ~Vm^{-1}$