a
\(\begin{array}{l}
{\rm{Distance}}\,travelled\,in\,the\,{3^{rd}}\,{\rm{second}}\\
 = {\rm{Distance}}\,travelled\,in\,3\,s\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - \,d{\rm{istance}}\,{\rm{travelled}}\,{\rm{in}}\,\,{\rm{2}}\,\,{\rm{s}}{\rm{.}}\\
As,\,u = 0,\\
{S_{\left( {{3^{rd}}s} \right)}} = \frac{1}{2}a{.3^2} - \frac{1}{2}a \cdot {2^2} = \frac{1}{2}a \cdot 5\\
Give\,a\frac{4}{5}m{s^{ - 2}};\,\,\,\therefore \,\,\,{S_{\left( {{3^{rd}}s} \right)}} = \frac{1}{2} \times \frac{4}{3} \times 5 = \frac{{10}}{3}\,m
\end{array}\)