According to Bohr's model of an atom, the 'M' shell is represented by the principal quantum number 'n' equal to:

Correct option is B

The correct answer is (b) 3

Explanation:
• Bohr's model of the fundamental atom formally suggests that negatively charged electrons rapidly revolve around the central nucleus in discrete, well-defined circular orbits or shells.
• These specific planetary-like orbits or shells are scientifically referred to as distinct energy levels.
• Conventionally, they are denoted by the alphabetical letters K, L, M, N... or sequentially by the principal quantum numbers n = 1, 2, 3, 4...
• The very first shell (which is situated closest to the positively charged nucleus) is logically the K shell (corresponding to n=1).
• Following the sequence, the second shell is the L shell (n=2), and the third consecutive shell is the M shell (n=3).

Information Booster:
• The absolute maximum number of electrons that can be physically accommodated in any specific energy shell is governed by the mathematical formula $2n^2$.
• Therefore, for the specific M shell (where n=3), the maximum theoretical electron capacity calculates perfectly to $2 \times (3)^2 = 18$ electrons.

Additional Knowledge:
2 (Option a)
• This specific principal quantum number strictly represents the L shell, which is the second shell from the nucleus.

4 (Option c)
• This specific principal quantum number correctly represents the N shell, which logically follows the M shell.

1 (Option d)
• This fundamental principal quantum number directly represents the K shell, which is the lowest possible energy level closest to the nucleus.

So the correct answer is (b)