Answer: Please find the answer in the explanation
Explanation: According to the Newton 3rd law of motion which state that;
In every action, there will be equal and opposite reaction.
when the piston of a fountain pen with a nib is dipped into ink and and the air is released by pressing it, a force is applied which act on the molecules of the ink by pushing the molecules. When the force is released, there will be a reaction force by pulling the molecules of the ink into the pen. Thereby the ink fills in the pen.