When we graph a linear equation we draw a line. When we want to graph a quadratic equation we draw a parabola, a parabola is a U-shaped curve. The graph of an exponential function grows or decays and has asymptotic values.
In this case, the first graph is a line, so it represents a linear equation.
The second graph is a U-shaped curve, then it represents a quadratic equation.
The third graph decays, then it represents an exponential equation.