Which of the following is a quadratic function?

a. y = x2 - 81

b. y = x4 - 64

c. y = x - 9

d. y = x3 - 27

a. y = x^2 - 81 is a quadratic function.

According to the definition:

A quadratic function is defined as a function with the maximum coefficient of 2. Only first option has a coefficient of 2 and the rest have not.