The quantum number which explain the line spectra observed as doublets in case of hydrogen and alkali metals and doublets and triplets in case of alkaline earth metals is:
A. Spin
B. Azimuthal
C. Magnetic
D. Principal

:Hint:Doublets are defined by the characters of electron spin. There are only two values defined for the electron spin which exists in a quantum state. These values are based on the orientation of the electrons.

Complete answer:
The doublet state mainly offers two specific values related to the spin component of the electrons. These two are $ + \dfrac{1}{2}$ and $ - \dfrac{1}{2}$ which are based on the orientation or the spin of the electron. Therefore, one is clockwise which is written as an upwards arrow and the other one is anticlockwise depicted by a downward arrow. In every orbital these spins of electrons are observed.
The doublets define the line spectra which is observed in the case of Group I elements, hydrogen and alkali metals. This is a mixed quantum system which has a specific spin of $\dfrac{1}{2}$ from where the possible values are derived.
The triplet state of the quantum has the specific spin of $1$ and there are three values allowed as a part of the spin component $ - 1,0, + 1$ . These are the values for the triplet state as observed in the case of alkali metals. The alkali metals have these triplet state as well as the doublet state.

Hence, the correct quantum number that explains the line spectra properly is A. spin quantum number.

Note:
The spin quantum number or the electron spin is directly associated with the duplet or the triplet state. The number of possible values for each of the states is responsible for its character.