Explanation: FCC regulations, specifically Part 13 governing commercial radio operators, mandate that any operator on duty and in charge of a transmitting system must make their license available for inspection. This requirement is met by either having the original license posted at the operating position or by having a photocopy in the operator's personal possession. This ensures that regulatory authorities can immediately verify an operator's qualifications. Option A is incorrect because the Proof-of-Passing Certificate (PPC) is used to obtain the license initially, but it is not a document required to be kept in station records by an operator on duty. Option C refers to FCC Form 605, which is primarily an application form for a license, not a document required on-site for daily operation. Option D describes a potential internal administrative requirement, but it is not the direct FCC regulatory requirement concerning license availability for inspection.