Final answer:
A private pilot must pay at least their pro rata share of the operating expenses when carrying passengers and may not seek compensation for the flight beyond this cost sharing.
Step-by-step explanation:
Regarding what a private pilot must pay for a flight with passengers, the FAA regulations stipulate that a private pilot may not act as pilot in command of an aircraft that is carrying passengers for compensation or hire. However, they can share the operating expenses of a flight with their passengers as long as the expenses involve only fuel, oil, airport expenditures, or rental fees, and as long as the pilot pays at least their pro rata share of the total expenses. Therefore, the correct answer is b) Pro rata share of expenses. This rule ensures that the pilot has a legitimate share in the costs of the flight and is not profiting from it, which would require a commercial pilot's license.