In the Millikan's Oil Drop experiment, the oil drop is subjected to such forces whose nature does NOT fall under the category of:

magnetostatic

The correct answer is option 4. magnetostatic.

In the Millikan's Oil Drop experiment, the oil drop is subjected to forces, but the nature of forces does NOT fall under the category of Magnetostatic.

In Millikan's Oil Drop experiment, the primary forces acting on the oil drop are:

1. Gravitational Force: This force acts downward on the oil drop due to gravity.

2. Viscous Drag Force: This force acts upward on the oil drop due to the resistance offered by the surrounding air or gas in the chamber.

3. Electrostatic Force: This force arises due to the presence of an electric field in the chamber. By applying an electric field, the oil drop can be made to move either upward or downward depending on the magnitude and direction of the electric field.

Magnetostatic forces, which are forces arising from magnetic fields, are not typically involved in Millikan's Oil Drop experiment. The experiment primarily focuses on studying the behavior of charged oil droplets in the presence of electric fields to determine the fundamental unit of electric charge (the charge of an electron). Therefore, magnetostatic forces are not relevant to this experiment.