B. (The explanation) Graph b. represents a quadratic function (since the function has an x^2 in it). The other functions are all linear because you multiply the variable x with an integer (or a fraction as seen in equation c.)
Note: Equation c. can be rewritten the following way to make clear it is a linear function. [tex]y = \frac{x}{2} + 1 = \frac{1}{2} x + 1[/tex]
