The spring extends by x on loading, then energy stored by the spring is :
(if T is the tension in spring and k is spring constant)

• A

  $\frac{{\mathrm{T}}^2}{2\mathrm{k}}$

• B

  $\frac{{\mathrm{T}}^2}{2{\mathrm{k}}^2}$

• C

  $\frac{2\mathrm{k}}{{\mathrm{T}}^2}$

• D

  $\frac{2{\mathrm{T}}^2}{\mathrm{k}}$