Please explain the process of finding the correct solution.

For the graph shown below, find all values of x for which f(x) is not differentiable. 6 -5 -2 2 3 5 6 1 .5 .6 a.) x= - 1, x = 0, X = 1 O b.) x= - 1, x=0 c.) f(x) is differentiable for all x-values. O d.) x = - 1