difference value = increased value - original value
rate of increase = $\left ( \frac{difference value}{original value} \right )100$
here if original consumption value = x
increased consumption value = 2x
difference value = 2x-x = x
so rate of increase = $\left ( \frac{x}{x} \right )100$ = 100%
but it is over span of 10 years. so annual rate of increase is 100/10 =10.0 (as increase is uniform over years)
so answer C