Final answer:
The fourth quantum number of a 2s² electron, representing the spin projection, could either be +1/2 or -1/2, known as the spin quantum number.
Step-by-step explanation:
The fourth quantum number of a 2s² electron could either be +1/2 or -1/2, which represents the spin projection quantum number (ms). This number determines the orientation of the electron's spin and can only have these two values for any electron. The possible choices for the spin quantum number (ms) of an electron are +1/2 (referred to as spin up) and -1/2 (referred to as spin down).