D. √5
Let's first simplify the equation
sqrt(75)/sqrt(15)
Factor it
(sqrt(3)sqrt(5)sqrt(5)) / (sqrt(3)sqrt(5))
Now cancel the sqrt(3) and sqrt(5) on both top and bottom, giving
sqrt(5) / 1
sqrt(5)
So we're just left with the square root of 5. Checking the available options, you see that D. √5 is an exact match. So that's your answer.