Given:
The power is lost in a reactor
To find:
What lowers the control rods into the core to shut it down
Step-by-step explanation:
Control rods are important safety features of reactors as they allow the user to control the output of the reactor. The control rods are held by electromagnetic fields.
If there is some sort of power failure or loss of signal the control rods are immediately released and fall into the reactor core because of gravity.
Hence, the correct option is gravity.