Answer:
B
Explanation:
a is a cubic. A cubic is an equation that has 3 on its base x to become x^3 as it's highest power. That is not a quadratic. A cubic and a quadratic are not the same thing.
b is a quadratic The highest power is 2 on the base (x) to become x^2. That is how a quadratic is defined.
c is a reciprocal. It has its highest power in the denominator, and that is not a quadratic. A quadratic is an equation whose highest power is 2 on its base (x) and whose highest power is in the numerator, not the denominator. This choice is rather tricky because the quadratic formula can be used on it if you change x = 1/z. Then the equation becomes f(x) = 4z^2 - 2z + 1. But as it is given and likely as you have been told, this is not a quadratic. This will factor into (2z - 1)(2z - 1). If I were a student, I'd question this one. It would be interesting to see how your instructor would eliminate it.
d is just a linear function. Since x^2 is multiplied by 0, it disappears. What is left is f(x) = -9x + 7 which is a linear function.