b
\(\overrightarrow F  = (2\hat i - 4\hat j + 2\hat k)\,N\) and \(\overrightarrow r \, = (3i + 2 - 4\hat k)\) \(meter\)

Torque \(\overrightarrow \tau  \, = \overrightarrow r  \times \overrightarrow F \)  \( = \left| {\,\begin{array}{*{20}{r}}{\hat i}&{\hat j}&{\hat k}\\3&2&{ - 4}\\2&{ - 4}&2\end{array}\,} \right|\)

\(⇒\) \(\overrightarrow \tau  \, =  - 12\hat i - 14\hat j - 16\hat k\) and \(|\overrightarrow \tau  |\, = \sqrt {{{( - 12)}^2} + {{( - 14)}^2} + {{( - 16)}^2}} \) = \(24.4\ N-m\)