As given in question (A) and (R) both are correct and (R) is the correct explanation of (A)
We know that for the electron in the same orbital, attraction is more for a larger atomic number. An increase of attraction decreases the energy of the orbital and the electron present in it.
Hence, the given reason in question is correct that energies of the orbitals in the same subshell decreases with an increase in the atomic number.
Taking into account the given elements, 2s−orbital (energy level) of H atom will have greater energy than that of 2s−orbital of Li− atom.