1. For an electromagnetic wave, the direction of propagation is given by the Poynting vector:

$\vec{S} \propto \vec{E} \times \vec{B}$

So,

$\vec{E} \times \vec{B} = \text{direction of wave propagation}$

2. Given:

• Wave is travelling in the negative $z$ direction
• Electric field $\vec{E}$ is along positive $y$ direction

Thus,

$\hat{j} \times \vec{B} = -\hat{k}$

3. Now use the standard cross-product relations:

$\hat{j} \times \hat{i} = -\hat{k}$

This matches exactly.

So,

$\vec{B} \text{ is along } +\hat{i}$

i.e. magnetic field is in the positive $x$ direction.

4. Checking options:

• A: Negative $y$ direction — incorrect
• B: Positive $z$ direction — incorrect
• C: Positive $x$ direction — correct
• D: Negative $x$ direction — incorrect

Therefore, the correct option is C.