Marcus has made a practice of asking his policyholders to pay their premiums at his agency instead of directly to the insurance company. Marcus then immediately forwards the premiums to the insurance company. This way, he can determine if there are any missing payments and contact policyowners directly if that should happen. His contract does not expressly grant this; however, the insurer is aware of the practice and has not asked him to stop. This practice is an example of:

a) Waiver
b) Estoppel
c) Subrogation
d) Representations