The graph of the function X = a + bY is

The graph of the function X = a + bY is linear. The equation is in the form of a straight line where X is the dependent variable and Y is the independent variable. The slope of the line is equal to the value of b, and the intercept of the line is equal to the value of a. As the value of Y increases or decreases, the value of X also changes proportionally according to the value of b. Therefore, the graph of the function is a straight line with a constant slope. Hence, the correct answer is "linear".