What is the primary role of the motor liner in a solid rocket motor?

The primary role of the motor liner in a solid rocket motor is to keep the hot gases produced during combustion from burning or melting the motor case. The motor liner serves as a thermal barrier, protecting the outer structure of the motor from the intense heat generated when the solid propellant ignites and burns. This is crucial because maintaining the integrity of the motor casing ensures the rocket functions safely and effectively during its flight. If the liner were not present, the heat could compromise the motor case's material properties, leading to catastrophic failure.

Other options address different aspects of motor performance or construction but do not capture the essential protective function of the liner. For instance, while enhancing structural integrity or reducing weight is important in rocket design, these are not the primary responsibility of the motor liner. Similarly, while a motor liner could unintentionally influence the burn rate of propellant, that is not its primary designed purpose. Thus, option B accurately identifies the critical protective role of the motor liner in ensuring the reliable operation of solid rocket motors.