Graph B is the only straight line, thus the linear function of x and the correct option is B.
A linear function is a function whose graph is a straight line. This means that for any two points on the graph, the change in y divided by the change in x is always the same.
Looking at graph B, we can see that the line is perfectly straight. This means that the change in y divided by the change in x is always the same, regardless of where we are on the line.
Therefore, graph B is a linear function of x.
Graphs A, C, and D are not linear functions of x. Graph A is a quadratic function, meaning that its graph is a parabola. Graph C is a piecewise function, meaning that its graph is made up of two different lines. Graph D is an exponential function, meaning that its graph curves upward.
Therefore, the correct option is B.