a
\(\begin{array}{l}
s = {t^3} + 5 \Rightarrow velocity,\,v = \frac{{ds}}{{dt}} = 3{t^2}\\
{\rm{Tengential}}\,acceleration\,{a_t} = \frac{{dv}}{{dt}} = 6t\\
Radial\,acceleration\,{a_c} = \frac{{{v^2}}}{R} = \frac{{9{t^4}}}{R}\\
At\,t = 2s,\,{a_t} = 6 \times 2 = 12\,m/{s^2}
\end{array}\)

\(\begin{array}{l}
{a_c} = \frac{{9 \times 16}}{{20}} = 7.2\,m/{s^2}\\
\therefore {\rm{Resultanl}}\,accelertaion\\
 = \sqrt {a_t^2 + a_c^2}  = \sqrt {{{\left( {12} \right)}^2} + {{\left( {7.2} \right)}^2}}  = \sqrt {144 + 51.84} \\
 = \sqrt {195.84}  = 14m/{s^2}
\end{array}\)