Which of the following is a condition that may require an aircraft to be grounded?

Grounding an aircraft is often necessary when it has experienced substantial damage that compromises its airworthiness. Substantial damage refers to any condition that significantly affects the aircraft’s structural integrity, performance, or systems, leading to safety concerns. This type of damage can impair critical functions or create hazards during operation, necessitating inspections, repairs, or thorough evaluations by qualified personnel before the aircraft can be safely operated again.

Other conditions, while potentially problematic, do not inherently require grounding. Minor cosmetic damage does not typically impact the functionality or safety of the aircraft. An expired registration is a legal issue that requires resolution for compliance but does not directly affect the aircraft's ability to fly safely. A low fuel level, while significant for operational planning, does not warrant grounding as the aircraft can be refueled before flight. Therefore, substantial damage that necessitates repair is the clear reason for grounding an aircraft, ensuring that it meets safety regulations and is fit for flight.