This question, in my opinion, is not well stated. If f(x) = √x, as the question statement seems to say, then the domain is not x<7. Rather, the domain is x≥0.
If f(x) is not the square root function, but say f(x) = √(7-x) then the domain is x≤7, and for this function then the appropriate answer is d), since the x-term inside the radical has a negative coefficient.
