Which of the Following Is Synonymous with a "Retainer" with Regard

Which of the following is synonymous with a "retainer" with regard to professional services?

A) the client's contractual obligation to provide accurate information for the professional opinion
B) a document setting out the terms of liability of the professional for professional negligence
C) a written promise of the professional to act in the best interests of the client
D) the advance payment requested by the professional to fund the professional's services