All we care about is what happens at, or near
#x =-1#
#f# is constant (its value doesn't change) near #x = -1#
No matter if we approach #-1# from the left or the right, #f# remains constant at #f(x) = 1#
Since #f# is also defined at that point, it holds that both one-sided limits (questions a,b) exist, are real numbers, and equal to #1#, so the general limit (two sided limit, question c) also exists and is #1#. Finally, judging from the graph, #f(-1)# is clearly defined, and equal to #1# (question d).
