B. y = 5x² represents a proportional relationship.
In a proportional relationship, the ratio of the dependent variable (y) to the independent variable (x) is constant. In this equation, as x increases, y also increases by a constant factor (5 times x). This indicates that the two variables are proportional to each other.
In A. y = 4x + 1 and C. y = 9, there is no constant ratio between y and x, so they do not represent proportional relationships.