In the context of a system and its surroundings, the surroundings refer to everything outside of the system itself. In this example, the system is the pot of stew. Therefore, the part of the surroundings would be:
The energy cooking the stew.
The other options (the pan, the stew, and the lid) are all part of the system or components of it, whereas the energy (usually from the heat source) is considered part of the surroundings.