Which scenario represents the weakest force? (assume particles in all scenarios are the same distance apart.)

protons and neutrons are held together inside the nucleus.
two particles are attracted to one another by static electricity.
an unstable atom breaks down releasing two radioactive particles.
two rocks of equal mass fall to the ground at the same speed.

Two rocks of equal mass fall to the ground at the same speed is the  scenario represents the weakest force.

Option D

Explanation:

There are four fundamental forces existing in the Universe and the weakest force is gravitational force amongst them. This force is also responsible for the particle to fell down on the earth because of its weakest attraction force as compared to the other forces applied on the object.

It is the force between two object masses and here given is at equal distance. So, when two rocks with equal speed and mass at a same distance should be the case of showing weakest force.