Final answer:
In quantum mechanics, 'n' represents the principal quantum number which determines the energy levels of an electron in an atom.
Step-by-step explanation:
The quantum number represented by 'n' in quantum mechanics is the principal quantum number. The principal quantum number (n) labels the energy levels or shells of an electron in an atom, with possible values of n = 1, 2, 3, and so on. It is instrumental in determining the energy of an electron and the size of the electron's orbit. The angular momentum quantum number (l) describes the shape of the electron's orbital and has values that range from 0 to (n - 1). The magnetic quantum number (m) describes the orientation of the orbital in space and is dependent on the value of l. The spin quantum number (s) describes the spin direction of the electron, which can be +1/2 or -1/2. Therefore, the correct answer to the question is (a) Principal quantum number.