The correct answer is D. all of the above.
A nickel-iron battery is a type of secondary battery that uses nickel hydroxide and iron as its active materials. The electrolyte is a 21% solution of potassium hydroxide.
The nickel hydroxide is the cathode material, and the iron is the anode material. When the battery is discharged, the nickel hydroxide is reduced to nickel oxide, and the iron is oxidized to iron oxide. When the battery is charged, the reverse reaction occurs.
Nickel-iron batteries are known for their long life, durability, and resistance to abuse. They are also relatively maintenance-free. However, they have a lower energy density than other types of batteries, such as lead-acid batteries.
Nickel-iron batteries are used in a variety of applications, including electric vehicles, forklifts, and backup power systems.