Question

nucleus of hydrogen is assumed to be fundamental particle of atom

Answer 1

neutrons + protons are fundamental particles present in an atom

Answer 2

Nucleus of hydrogen atom is considered to fundamental particle of atom because hydrogen atom is the simplest possible atom with only one proton in its nucleus and one electron around it, the absence of neutron makes it as the fundamental particle of atom.

One of the lightest atom is considered as the hydrogen nucleus Rutherford hence thought that hydrogen is the fundamental particle of a nucleus and a lightest atom.