Which of the following functions is quadratic?
a. f(x)= 5/x-1
b. f(x)=2^2
c. 7x(x+3)
d. f(x)= 3(4-x)

The answer is C. 
The definition of a "quadratic equation" is any equation that has a variable with an unknown value to a power no less than or more than 2. A is not the answer because nothing is to the 2nd power. B, although is to the second power, is simplified into just 4, so it has neither an unknown variable nor power of 2. D is also incorrect because when the 3 is distributed, the equation is just simply 12-3x, which has no power of 2. So C is correct because when the 7x is distributed, you get the equation 7x^2 +21x, which has both a variable with an unknown value and a power of two.