The correct answer is D. The rent earned by a factor of production is equal to the difference between what this factor is currently earning and what this factor can obtain in its next best use.
Option A is incorrect because the rent earned by a factor of production is not equal to what this product can earn in its next best use. For example, if a farmer is currently earning $100 per acre for growing corn, and the next best use for that land is to grow soybeans, and the farmer could earn $90 per acre for growing soybeans, then the rent earned by the land is $10 per acre.
Option B is incorrect because the rent earned by a factor of production is not equal to the sum of this output accrues in its present use and what it can earn in its next best use. For example, if a farmer is currently earning $100 per acre for growing corn, and the next best use for that land is to grow soybeans, and the farmer could earn $90 per acre for growing soybeans, then the rent earned by the land is $10 per acre, not $190 per acre.
Option C is incorrect because the rent earned by a factor of production is not equal to his present earnings. For example, if a farmer is currently earning $100 per acre for growing corn, and the next best use for that land is to grow soybeans, and the farmer could earn $90 per acre for growing soybeans, then the rent earned by the land is $10 per acre, not $100 per acre.