(c) Energy stored= work done
So $E = \frac{1}{2}kr^{2}$
or $r = \sqrt{\frac{2E}{k}} = \sqrt{\frac{2 \times 2}{400}} = \frac{1}{10}\ m$
$a = \omega^{2}r = \left( \sqrt{\frac{k}{m}} \right)^{2} \times \frac{1}{10}$
$= \left( \frac{400}{1} \right) \times \frac{1}{10} = 40\ ms^{- 2}$