Evaluate Assertion A

Assertion A says:

Energy of $2s$ orbital of hydrogen atom is greater than that of $2s$ orbital of lithium.

For hydrogen-like understanding, orbital energies become more negative as nuclear charge increases.

• In hydrogen, the $2s$ electron feels nuclear charge of only $+1$.
• In lithium, the $2s$ electron is attracted by a higher nuclear charge, so its energy is lowered (made more negative), even after shielding.

Hence, $E_{2s}(\text{Li}) < E_{2s}(\text{H})$ so the energy of $2s$ orbital in hydrogen is indeed greater than that in lithium.

Therefore, Assertion A is true.

Evaluate Reason R

Reason R says:

Energies of the orbitals in the same subshell decrease with increase in the atomic number.

This is generally true: for a given type of orbital such as $2s$, increasing atomic number increases effective nuclear attraction, so orbital energy decreases (becomes more negative).

Thus, Reason R is true.

Check whether R explains A

Assertion A compares the $2s$ orbital energies of hydrogen and lithium.

Since lithium has higher atomic number than hydrogen, by the statement in R, the energy of the $2s$ orbital should decrease as atomic number increases. Therefore, $E_{2s}(\text{Li}) < E_{2s}(\text{H})$ which directly explains Assertion A.

So R is the correct explanation of A.

Final choice

Therefore, the correct option is: $\boxed{\text{A}}$

Comparison with stored answer

Stored correct answer = A.

My derived answer also = A, so they agree.