In this worksheet you must decide whether a circuit will work. For example, whether it will light a bulb or make a fan work.
As you know, electric current flows because it is pushed by cells (or batteries). For the current to flow right around a circuit, there must be no gaps in it.
If the circuit has a switch in it, and the switch is open, that's a gap: no electric current will flow.
The green component in this circuit is a type of switch called a reed switch. Reed switches are switched on by magnets - bringing a magnet near to it will close the switch and allow electric current to flow.
So, check whether a circuit is complete and whether the current can get round it.
Remember: the higher the electric current, the brighter any lamps will be, the faster a motor will turn and the louder a buzzer will sound.