Final answer:
In NiCad rechargeable batteries and nuclear power plants, the metallic element used is cadmium (Cd). Therefore, correct option is a.
Step-by-step explanation:
The metallic element used in nuclear power plants and in NiCad rechargeable batteries is cadmium (Cd). NiCad, which stands for nickel-cadmium, is a type of rechargeable battery notable for its use in small electrical appliances and devices. These batteries contain a cadmium anode and a highly oxidized nickel cathode.
The water-based cell of the NiCad battery aims to maximize the surface area of the electrodes and minimize the distance between them, reducing internal resistance and enabling high discharge current.
Cadmium is also utilized in nuclear technology due to its ability to absorb neutrons. However, it is important to note that cadmium is a toxic heavy metal and requires proper disposal in accordance with toxic waste guidelines to avoid environmental contamination.