Which of the following describes an open system?

An open system is characterized by its ability to exchange both matter and energy with its surroundings. This means that within an open system, materials can flow in and out freely, contributing to various processes such as chemical reactions or physical transformations. For instance, consider a pot of boiling water with its lid off. Water vapor can escape into the air (matter leaving), while heat from the burner warms the water (energy entering).

In contrast, a system that cannot lose energy is a closed system. While it allows energy transfer, it does not permit the exchange of matter. A system that only allows energy to exit is not a typical classification in thermodynamics, since that would still imply some restrictions on matter and is not consistent with the definition of open systems. Finally, a completely isolated system would not interact with its environment at all, meaning no exchange of both energy and matter happens, which is the opposite of an open system.

Therefore, the correct answer captures the essential definition of an open system, highlighting its interaction with the external environment.