Answer
$(A)\ 0.766mJ$
Work Step by Step
We know that
$F=\frac{K}{x}$
$\implies K=\frac{F}{x}$.......eq(1)
Now $slope=x=\frac{4.0mm-0}{0.5N-0}=8.0\frac{mm}{N}$
We plug in the known values in eq(1) to obtain:
$K=\frac{1.0}{8.0mm\times \frac{1m}{1000mm}}=125\frac{N}{m}$
Now we can find the energy stored in the forewing as
$U=\frac{1}{2}Kx^2$
We plug in the known values to obtain:
$U=\frac{1}{2}(125)(0.0035)^2$
$U=7.66\times 10^{-4}J=0.766mJ$
