Chapter 5 - Work and Energy - Learning Path Questions and Exercises - Multiple Choice Questions - Page 172: 6

(c) $W=\frac{1}{2}kx^2$

Work done by a force $F=kx$ is analogous to the work done by a uniformly variable spring force; given by the area under an F-versus-x plot. Since, it forms a straight line, area under the curve is given by the area of the triangle (Figure 5.6). Work done, $W=\frac{1}{2}(kx)(x)=\frac{1}{2}kx^2$