Select the correct statement(s) among the following. 

Options

A. The maximum value of principal quantum number is 7
B. For n = 2, there may be four subshells and these may contain a maximum of 8 electrons.
C. M shell can accommodate a maximum of 18 electrons.
D. The energy of 5p-orbitals is more than that of 4d-orbitals.

Answer

To solve the question of selecting the correct statements regarding the structure of an atom, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: The maximum value of principal quantum number is 7.**
- The principal quantum number (n) indicates the number of shells in an atom. While it is common to refer to n values up to 7 in many contexts, theoretically, there is no upper limit to n; it can be infinite. Therefore, this statement is **incorrect**.

2. **Statement 2: For n = 2, there may be 4 sub shells.**
- For n = 2, the possible values of the azimuthal quantum number (l) are 0 and 1 (l can take values from 0 to n-1). This means there are 2 sub shells: 2s (l = 0) and 2p (l = 1). Thus, the statement claiming 4 sub shells is **incorrect**.

3. **Statement 3: M shell can accommodate a maximum of 18 electrons.**
- The M shell corresponds to n = 3. The sub shells in the M shell are 3s, 3p, and 3d. The maximum number of electrons in these sub shells is:
- 3s: 2 electrons
- 3p: 6 electrons
- 3d: 10 electrons
- Total = 2 + 6 + 10 = 18 electrons. Therefore, this statement is **correct**.

4. **Statement 4: The energy of 5P orbital is more than that of 4D orbital.**
- To compare the energies, we use the n + l rule:
- For 5P: n = 5, l = 1 → n + l = 5 + 1 = 6
- For 4D: n = 4, l = 2 → n + l = 4 + 2 = 6
- Since both have the same n + l value, we compare the principal quantum numbers. The 5P orbital has a higher principal quantum number (n = 5) than the 4D orbital (n = 4), indicating that the 5P orbital is at a higher energy level. Therefore, this statement is **correct**.

### Conclusion:
The correct statements are:
- Statement 3: M shell can accommodate a maximum of 18 electrons.
- Statement 4: The energy of 5P orbital is more than that of 4D orbital.

### Final Answer:
The correct options are **3 and 4**.

---