b
\(\overrightarrow{ r }=3 t \hat{ i }+5 t ^{3} \hat{ j }+7 k\)

\(\frac{ d ^{2} \overrightarrow{ r }}{ dt ^{2}}=30 t \hat{ j }\)

At \(t=1 \Rightarrow \frac{d^{2} \overrightarrow{r}}{{d t^{2}}}=30\,\hat{j}\)