We have been given a piecewise function such that:
When x is less than 1, the value of the function f(x) is equal to x. Which means the when x =0, f(x) = 0, when x= -1, f(x) = -1 , when x=-5, f(x) = -5, and so one, as far as x is less than one. It means that the graph of this part is a linear increases/decreases which show the relationship bet f(x) and y.
When x is equals to or greater than 1, the value of f(x) becomes constants that is 1. Hence, at x=1 and after that, the graph neither increases or decreases, as it becomes a straight line at y=1.
Graph of the piecewise function is shown, it matches option D.