The correct answer is a - it is continually replenished.
Renewable energy is said to be any energy source that is naturally replenished , such as that derived from the sun (solar), wind, geothermal or hydroelectric action. Also energy produced from refining of biomass is often placed in the class of renewable energy.
Renewable energy source is from an energy source that is replaced by a natural process at a rate that is equal to or faster than the rate at which that resource is being consumed.
Solar energy handsomely fulfils these descriptions. It is natural, inexhaustible and more than enough to sustain the earth for millions of years yet.