According to the second law of thermodynamics, which is a requirement for thermal energy to flow from colder objects to hotter objects? Thermal expansion must occur in the system. Convection must occur within the system. Work must be done on the system. Radiation must alter the system.

Work must be done on the system

Explanation:

The 2nd law of thermodynamics states that the entropy of an isolated system can never decrease over time. A direct consequence of this law is that thermal energy (heat) can spontaneously flow only from hotter objects to colder objects, but never in the opposite direction. In fact, when two objects at different temperatures are put in contact, the hotter one transfers heat to the colder one: in this way, the entropy of the system increases (because the heat is 'spread' over the two objects). The opposite process would violate the 2nd law of thermodynamics.

The only way to transfer heat from a colder object to a hotter objects is by doing external work on the system. This is essentially what is done in a refrigerator: by doing external work, the refrigerator transfers heat from a colder environment (the interior of the fridge) to a hotter environment (the surrounding).