Maximums of Functions

A function has a maximum when there is one point on the graph that has the highest y-coordinate. Determine if the following functions have a maximum, and if so, under what circumstances does the maximum occur? Explain your reasoning.
a. Linear: y = ax + b
b. Quadratic: y = ax² + bx + c
c. The function y = axⁿ where n is an even positive integer
d. The function y = axⁿ where n is an odd positive integer