:8b.  Solution.  Since the average of the list -1, 3, 6, 9, 13
is 6, the deviations from the average are -7, -3, 0, 3, 7.  The
squares of these deviations are 49, 9, 0, 9, 49.  The average of
the squares is (49+9+0+9+49)/5 = 114/5 = 22.8.  The square root
of 22.8 is 4.77.  The closest of the answers is 5.