If an alteration or repair substantially affects an aircraft's operation in flight, that aircraft must be test flown by an appropriately-rated pilot and approved for return to service prior to being operated:

• A. with passengers aboard
• B. with the pilot's friend aboard
• C. only during maintenance flight tests
• D. without passengers or crew aboard

Answer: (A)

When an alteration or repair has the potential to change how the aircraft flies, you must prove in flight that the modification doesn’t compromise safety. This is done by a flight test conducted by a pilot who holds the appropriate rating for that aircraft, followed by an official return-to-service authorization. Only after that authorization is in place may the aircraft be operated again in service, including flights with passengers on board. The purpose is to verify that handling, performance, and systems remain within approved limits after the change.

So the key idea is that the aircraft must be tested and formally cleared before it can carry passengers again.