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 correct answer is A) Gravity is the strongest of the four. This statement is incorrect. In reality, the strong nuclear force is the strongest of the four fundamental forces, followed by electromagnetism, the weak nuclear force, and finally, gravity, which is the weakest.

Let's analyze why the other options are correct:

B) Electromagnetism acts only on particles with an electric charge: This is true. Electromagnetism governs the interaction of charged particles. Neutral particles are not affected by electromagnetic forces.

C) Weak nuclear force causes radioactivity: This is also correct. The weak nuclear force is responsible for certain types of radioactive decay, like beta decay.

D) Strong nuclear force holds protons and neutrons inside the nucleus of an atom: This is accurate. The strong nuclear force overcomes the electrostatic repulsion between protons, binding them together with neutrons within the atomic nucleus.

Therefore, option A is the only incorrect statement.