What is the correct answer and why? concise and simple explanation !

(-∞, α)  

Step-by-step explanation:

The domain is the set of all possible x-values that will make the function work.

f(x) will exist only if the radicand is positive.

Also, x cannot equal α, because f(α) = 1/√0 =1/0 = undefined. Thus, we must have

α - x > 0 or

x < α

The domain is x <α.

In interval notation, the domain is (-∞, α).