The correct answer is B) a thermal trip unit. Molded-case circuit breakers are designed with a thermal trip unit to provide protection against sustained overloads. The thermal trip unit uses a bimetallic strip that heats up and bends when subjected to an overcurrent condition, causing the circuit breaker to trip and open the circuit. This thermal overload protection is a key feature that distinguishes molded-case circuit breakers from other types, such as those that use magnetic trip units for instantaneous short-circuit protection. The other options are incorrect because they do not directly address the thermal protection against sustained overloads. Magnetic trip units respond to high, instantaneous current levels, reverse current relays detect reverse power flow, and overvoltage releases are triggered by excessive voltage, rather than overcurrent conditions.