b
\(\begin{gathered}
Work\,done = \left[ {\int\limits_0^2 {F.dx} = \,\int\limits_0^2 {kx\,dx\, = \frac{1}{2}k{x^2}} } \right]_0^2 \hfill \\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\frac{1}{2}.1.\left( {4 - 0} \right) = 2\,J \hfill \\
\end{gathered} \)