What is the composition of nucleus? 

Protons and neutrons 

The correct answer is option 1. Protons and neutrons.

The nucleus is the central and most significant part of an atom. It contains protons and neutrons, which are the subatomic particles with mass. Here's a brief explanation:

1. Protons:  Positively charged particles found in the nucleus. The number of protons is the atomic number of an element, determining its identity.

2. Neutrons: Neutrally charged particles located in the nucleus. They contribute to the mass of the atom but not to its charge.

The nucleus, being positively charged due to protons, is surrounded by negatively charged electrons in various energy levels or electron shells. The electrons move in orbit around the nucleus, maintaining the overall electrical neutrality of the atom.

The composition of protons and neutrons in the nucleus determines the atomic mass of an atom. In summary, the nucleus is crucial in defining the properties of an element and plays a fundamental role in the structure of an atom.