Which of the following indicates that the roots of a quadratic are irrational
a) discriminant is zero
b) discriminant is negative
c) discriminant is rational
d) discriminant is prime

Answer would be D  You can have a rational discriminant for an irrational root, but you can also have rational discriminant for a rational root. So C is partially correct. 

All prime discriminants will result in an irrational root, but not all irrational roots have prime discriminants