Your Answer is

Correct Answer is C

Explanation

A unique solution to a quadratic equation means that the discriminant is equal to 0. So m²-4n=0. Substitute x=-3 into the equation to get (-3)²-3m+n=0, and n=3m-9. Substituting n into m²-4(3m-9)=m²-12m+36=0, the solution is m=6. So choose C.