Final answer:
Cadmium (Option B) is the metallic element used in nuclear power plant control rods to adjust neutron levels for safe chain reactions and is also used in Ni-Cad rechargeable batteries.
Step-by-step explanation:
The metallic element used in nuclear power plant control rods and in Ni-Cad rechargeable batteries is Cadmium (Option B).
Nuclear reactors use control rods to control the fission rate of the nuclear fuel by adjusting the number of slow neutrons present to keep the rate of the chain reaction at a safe level.
Control rods are often made of materials that are able to absorb neutrons, such as boron, cadmium, hafnium, or other elements.
Cadmium, in particular, is known for its neutron-absorbing properties, making it useful in control rods. Additionally, cadmium is used in Ni-Cad (nickel-cadmium) rechargeable batteries for its favorable electrochemical properties.