Protons and neutrons are bound in a nucleus by the `strong interaction`, which is the correct answer. The strong interaction, also known as the strong nuclear force, is one of the four fundamental forces in nature. It is responsible for holding the atomic nucleus together.
To understand the options, let`s examine them one by one.
- Option 1: Short-range `weak interaction` is incorrect because the weak interaction is responsible for processes such as radioactive decay, not for binding protons and neutrons in the nucleus.
- Option 3: Long-range `electromagnetic interaction` is incorrect because the electromagnetic force primarily affects charged particles, while protons and neutrons are not directly affected by it in the nucleus.
- Option 4: Long-range `gravitational interaction` is also incorrect because the gravitational force is extremely weak compared to the strong nuclear force and does not play a significant role in holding the nucleus together.
In conclusion, the strong interaction is the fundamental force that binds protons and neutrons together in the nucleus.