The magnetic field of a plane electromagnetic wave in vacuum is...

The argument of the cosine function is $(ky + \omega t)$, which indicates that the wave propagates in the negative y-direction. Thus, statement (A) is correct.

The direction of propagation of an electromagnetic wave is given by the direction of $\hat{E} \times \hat{B}$. Here, the propagation direction is $-\hat{j}$ and the magnetic field is along $\hat{i}$.
Let the electric field direction be $\hat{E}$. We require $\hat{E} \times \hat{i} = -\hat{j}$.
Since $(-\hat{k}) \times \hat{i} = -\hat{j}$, the electric field oscillates along the negative z-direction. Thus, statement (B) is correct.

The amplitudes of the electric and magnetic fields in vacuum are related by $E_0 = c B_0$. Statement (C) is incorrect.

In a plane electromagnetic wave, the electric and magnetic fields oscillate in phase. Thus, statement (D) is correct.

Therefore, the correct combination is (A), (B) and (D).

Answer: (A), (B) and (D) only