Answer:
(b) [the graph of a straight line]
Explanation:
A function is linear if a constant change in input (x) produces a constant change in output (y). When written as a polynomial equation, the exponents of all of the variables must be exactly 1.
The answer choices can be considered as follows:
a) exponent of 2 indicates nonlinear
b) the graph is of a straight line, a linear function
c) exponents of 2 indicate nonlinear
d) x-changes are constant, but y-changes are not, indicating nonlinear