If the discriminant of an equation is zero, which of the following is true of the equation? A. It has two complex solutions. B. It has two real solutions. C. It has one complex solution. D. It has one real solution.

Step 1 ：The discriminant of a quadratic equation is the part under the square root in the quadratic formula, i.e., \(b^2 - 4ac\).

Step 2 ：If the discriminant is zero, then the equation has exactly one real solution.

Step 3 ：This is because the square root of zero is zero, so the equation becomes \(-b/2a\), which is a single real number.

Step 4 ：\(\boxed{\text{D. It has one real solution}}\)