The correct answer is: A. An excess not a cost.
Economic rent is a payment to a factor of production in excess of what is necessary to keep it in its current use. It is a surplus that arises when the supply of a factor is fixed and the demand for it increases.
Option B is incorrect because economic rent is not a cost. It is a surplus that arises when the supply of a factor is fixed and the demand for it increases.
Option C is incorrect because economic rent is not always only one excess/surplus and one cost. It can be either an excess or a cost, depending on the circumstances.
Option D is incorrect because economic rent is both an excess and a cost. It is an excess because it is a surplus that arises when the supply of a factor is fixed and the demand for it increases. It is a cost because it is a payment that must be made to a factor of production in order to keep it in its current use.