Refer to figure.
In the diagram you can see the most basic type of thermal circuit breaker.

Thermal circuit breakers consist of two different types of metals, that are specifically chosen for their expansion rates for a certain temperature. Usually the top metal (shown in red) is copper, and the bottom metal (shown in blue) is iron.

As the current exceeds a set maximum, the metals will start to expand due to the excess heat. Now, because of the different expansion rates of the materials, the metal strip will bend and disconnect from the circuit, hence preventing anymore current from flowing.

Why is the correct answer “protects the system in the event of a prolonged overcurrent”:

• The overcurrent needs to be prolonged so that the materials have time to heat up and start expanding. Usually when an overcurrent occurs, the current is so much higher than the maximum current that it takes very little time (under a second) for the circuit breaker to trip. However, low over-currents can take minutes to cause the breaker to trip.
• The correct answer needs to contain overcurrent because thermal circuit breakers “protect” from overcurrent, they are not designed to trip at a specific surrounding air temperature. Even if a high temperature could trip the circuit breakers!