The correct option is
D We know the representation of a linear function on the graph is a straight line.
Option A:
The given graph is a straight line.
And as the graphical representation of any linear function is a straight line, so it is a linear function.
Option B:
The given graph is a straight line.
And as the graphical representation of any linear function is a straight line, so it is a linear function.
Option C:
The given graph is in the combination of multiple lines. It is not a single straight line.
And as the graphical representation of any linear function is a straight line, so it is a nonlinear function.
Option D:
The given graph is a straight line.
And as the graphical representation of any linear function is a straight line, so it is a linear function.
So option C is the correct answer.