a
(a)\({E_x}\) and \({B_y}\) would generate a plane EM wave travelling in \(z-\) direction. \(\vec E\), \(\vec B\) and \(\vec k\) form a right handed system \(\vec k\) is along \(z-\) axis. As \(\hat i \times \hat j = \hat k\)
==> \({E_x}\hat i \times {B_y}\hat j = C\hat k\) i.e. \(E\) is along \(x-\) axis and \(B\) is along \(y-\) axis.