The known forces of nature can be divided into four classes, viz., gravity, electromagnetism, weak nuclear force and strong nuclear force. With reference to them, which one of the following statements is not correct?

The known forces of nature can be divided into four classes, viz.,…

Answer

(a) Gravity is the strongest of the four — Incorrect. Gravity is actually the weakest fundamental force, though it dominates at large astronomical scales because it is always attractive and long-range.
(b) Electromagnetism acts only on particles with an electric charge — Correct. Electromagnetic force acts on electrically charged particles; neutral particles are not directly affected.
(c) Weak nuclear force causes radioactivity — Correct. The weak force is responsible for processes like beta decay, a common form of radioactivity.
(d) Strong nuclear force holds protons and neutrons inside the nucleus of an atom — Correct. The strong force binds quarks into protons/neutrons and also holds the nucleus together by overcoming proton-proton repulsion.