When does the square root of "n" represent a rational number?

Question

a. n is a perfect square
b. n is a positive integer
c. n can be written as a ratio
d. n can be any real number

Which is it and please explain.

Answer 1

n is a perfect square means that there is a rational number k such that k*k = n

Then k is the square root.

The others are clearly false, since
√5 is not rational
√(3/5) is not rational
Therefore, certainly not (d)