Which of the following statements best describes the nature of depreciation?

a. Regular reduction of asset value to correspond to the decline in market value as the asset ages.

b. A process of correlating the book value of an asset with its gradual decline in physical efficiency.

c. Allocation of cost in a manner that will ensure that plant and equipment items are not carried on the balance sheet at amounts in excess of net realizable value.

d. Allocation of the cost of a plant asset to the periods in which benefits are received.