Which of the following cell is a secondary cell?

Ni cell

The answer is 4. Ni cell.

A secondary cell is a type of electrochemical cell that can be recharged. This means that the cell can be used to produce electricity and then be recharged so that it can be used again.

Dry cells, mercury cells, and fuel cells are all primary cells. This means that they cannot be recharged.

Ni cells, also known as nickel-cadmium cells, are secondary cells. They are commonly used in portable devices such as laptops, cameras, and power tools.