Which of the following statements correctly describes the concept of a compensatory balance?

a) It is the difference between the balance in the cash account and the combined balances in all cash equivalent accounts.
b) It is the amount by which the chequing or savings accounts may be overdrawn.
c) It is the maximum amount that may be transferred between accounts.
d) It is the minimum cash amount that banks often require customers to maintain in their chequing or savings accounts