For the electron in the same orbital, attraction is more for larger atomic number. Increase of attraction decreases the energy of the orbital and the electron present in it.
Or the given elements, increase of atomic number is in the order, H < Li < Na < K
Energies of the orbitals in the same subshell decrease with increase in the atomic number.
Hence, 2s orbital (energy level) of potassium will have minimum energy than others.