What is the nucleus of H-atom known as? 

Proton 

The correct answer is option 3. Proton.

The nucleus of an atom is the central core that contains most of the atom's mass. In the case of a hydrogen atom, which is the simplest atom with only one proton in its nucleus, the nucleus consists solely of a single proton. Here's a more detailed breakdown:

1. Proton: A proton is a subatomic particle with a positive electric charge. It is found in the nucleus of an atom alongside neutrons. The number of protons in the nucleus determines the element's identity. In the case of hydrogen, there is only one proton in the nucleus.

2. Neutron: Neutrons are another type of subatomic particle found in the nucleus of atoms. They have no electric charge, unlike protons, but they contribute to the mass of the nucleus. While some isotopes of hydrogen do contain neutrons (such as deuterium and tritium), the most common isotope, protium, consists only of a single proton.

3. Electron: Electrons are negatively charged subatomic particles that orbit the nucleus of an atom in electron shells. However, in the context of the nucleus of an atom, electrons are not present. They are found outside the nucleus, moving in electron orbitals.

4. Nucleons: Nucleons are particles found in the nucleus of an atom. They include both protons and neutrons. In the case of a hydrogen nucleus, the term "nucleons" would technically refer to just one proton.

In summary, the nucleus of a hydrogen atom consists solely of a single proton, which is the positively charged subatomic particle responsible for defining the element hydrogen.