I assume your issue is why "mains" is used over "main".
When you talk about the "main power supply" of a circuit, you'll give most people connotations that there are multiple power supplies (e.g. batteries or cells), potentially in different places, and you're talking about the one that provides the most power.
The mains refers to electricity from an electrical grid that is connected to houses, businesses etc.
If you were to plug something into a wall's plug socket, this will likely use the mains power supply.
If you were to plug something into a standalone battery, like we used to do in our physics practicals, this would be the main power supply (or in our case, the only power supply).
They might not penalise in this specific question as mains isn't underlined, but I believe GCSE physics does expect you to know about the mains power supply.