a
\(\mathop L\limits^ \to = \mathop r\limits^ \to \times \mathop p\limits^ \to = \) \(\left| {\,\begin{array}{*{20}{c}}{\hat i\,\,}&{\hat j\,\,}&{\,\,\hat k}\\{1\,\,}&{\,2\,\,}&{ - 1}\\{3\,\,}&{\,4\,\,}&{ - 2}\end{array}\,} \right|\) \( = 0\hat i - \hat j - 2\hat k = - \hat j - 2\hat k\) and the X- axis is given by \(i + 0\hat j + 0\hat k\)
Dot product of these two vectors is zero \(i.e.\) angular momentum is perpendicular to \(X\) - axis.