Function f(x) is a quadratic equation and function g(x) is a linear equation. (Correct choice: D)
How to determine the nature of two functions
In this problem we find two functions, one in the form of a equation and other in the form of a table. The first equation is clearly a quadratic equation, that is, an equation of the form:
f(x) = a · x² + b · x + c
Where a, b, c are real coefficients.
And the second function is a linear equation if and only if slope is the same for every pair of points. This can be checked by means of secant line formula:

Now we proceed to check the condition:

m₁ = 4

m₂ = 4
m₁ = m₂ = 4
Thus, g(x) is a linear equation.