1. Key fact for hydrogen atom

For a hydrogen atom, the energy of an orbital depends only on the principal quantum number $n$.

$E_n = -\frac{13.6}{n^2}\ \text{eV}$

So, all orbitals having the same $n$ are degenerate (same energy):

• $2s = 2p$
• $3s = 3p = 3d$
• $4s = 4p = 4d = 4f$, etc.

Also, as $n$ increases, energy increases:

$1s < 2s=2p < 3s=3p=3d < 4s$

2. Check each statement

(A) $1s < 2p < 3d < 4s$

For hydrogen atom:

• $2p$ belongs to $n=2$
• $3d$ belongs to $n=3$
• $4s$ belongs to $n=4$

Thus,

$1s < 2p < 3d < 4s$

is correct.

(B) $1s < 2s = 2p < 3s = 3p$

For hydrogen atom:

• $2s = 2p$
• $3s = 3p = 3d$

So the given order

$1s < 2s = 2p < 3s = 3p$

is correct.

(C) $1s < 2s < 2p < 3s < 3p$

In hydrogen atom, $2s$ and $2p$ have equal energy. So writing

$2s < 2p$

is incorrect. Hence (C) is not correct.

(D) $1s < 2s < 4s < 3d$

For hydrogen atom, energy depends only on $n$:

• $3d$ has $n=3$
• $4s$ has $n=4$

Therefore,

$3d < 4s$

not $4s < 3d$. So (D) is not correct.

3. Identify the incorrect statements

The statements which are not correct are:

• (C)
• (D)

So the correct option is:

$\boxed{\text{D: (C) and (D) only}}$

4. Comparison with stored answer

Stored correct answer: D

Our derived answer: D

They match.