Explanation

(d) $\overrightarrow{r} = 3t\widehat{i} - t^{2}\widehat{j} + 4\widehat{k}$
Velocity, $\overrightarrow{v} = \frac{d\overrightarrow{r}}{dt} = \frac{d}{dt}\left( 3t\widehat{i} - t^{2}\widehat{j} + 4\widehat{k} \right) = 3\widehat{i} - 2t\widehat{j}$
At $t = 5s \Rightarrow \overrightarrow{v} = 3\widehat{i} - 10\widehat{j}$
$\left| \overrightarrow{v} \right| = \sqrt{(3)^{2} + ( - 10)^{2}} = \sqrt{9 + 100} = \sqrt{109} = 10.44\ ms^{- 1}$