Key concept: "for all x CLUE"
Rearrange the data: n>(something) for all x.
Then look for the only choice present n>(somthing): choice A

We can use the idea of discriminants to easily solve this question. Since the function must be >0, the discriminant can have no real roots (i.e. the parabola can never touch the x axis, if it touches the x axis, y=0, and is if its below the x axis, y will be negative, which is also not allowed)
We get no roots when b^2 -4ac is negative. Now, we can plug in the values of x and see where this holds. This occurs at all values of n>17 (when n is 17, the discriminant is 0 and we still have one root)

What helps me is to combine the n-13 into a value, say n, and solve it in the following way:
b^2 - 4ac--> 16-4m<0; 16<4m; m>4
n-13>4
n>17

discriminants can be very useful in these abstract quadratic type questions, don't sleep on them!!