**Explanation:**
The maximum number of electrons that can be accommodated in a shell is given by the formula 2n², where 'n' is the principal quantum number of the shell.

In the case of the third shell, n = 3. Therefore, the maximum number of electrons that can be accommodated in the third shell is:

2(3)² = 2(9) = 18

**- Electron distribution in shells:**

In an atom, electrons are distributed in different shells based on their energy levels. The shells are numbered starting from the innermost shell, which is designated as the first shell (n = 1), followed by the second shell (n = 2), and so on.

Each shell can hold a maximum number of electrons based on its principal quantum number (n). The electron distribution in shells is as follows:

- **First shell (n = 1):** This shell can hold a maximum of 2 electrons.

- **Second shell (n = 2):** This shell can hold a maximum of 2(2)² = 8 electrons.

- **Third shell (n = 3):** This shell can hold a maximum of 2(3)² = 18 electrons.

- **Fourth shell (n = 4):** This shell can hold a maximum of 2(4)² = 32 electrons.

- **Fifth shell (n = 5):** This shell can hold a maximum of 2(5)² = 50 electrons.

- **And so on...**

It is important to note that as the principal quantum number increases, the maximum number of electrons that can be accommodated in a shell also increases. This is because higher energy levels can accommodate more electrons.

Thus, the correct answer is option 'C' (18) as the maximum number of electrons that can be accommodated in the third shell (n = 3) is 18.