Which of the following is a criterion for a Child Restraint System (CRS)?

The criterion that a Child Restraint System (CRS) must fit in an aircraft passenger seat is important for ensuring safety and compliance with regulations. A CRS must be designed to be secured to a passenger seat so that it can adequately protect a child during the flight in the event of turbulence or an accident. Aircraft seating is made to accommodate standard-sized car seats or other CRS devices, allowing for proper installation and use as intended. This consideration prioritizes the safety of young passengers as well as ensuring that the aircraft's seating layout can properly function with these systems in place.

The other options do not specifically pertain to the fundamental requirements for CRSs in terms of safety and operational compatibility with the aircraft's seating configuration.