Remember to see the points of intersection of the graphs test for discriminant [b^2 - 4ac]
e.g. you have y = x^2 - 6x + 9, compare this to the general quadratic equation y=ax^2 + bx + c
Discriminant = (-6)^2 - 4(1)(9) = 0
For the graph to only touch x axis at one point discriminant must be equal to 0 so D is the answer as it is the only equations whose discriminant is equal to 0
