Tragedy of the commons clearly is that each herdsman would benefit from the addition of an extra animal graze around, but the cumulative loss of grazing potential of the pasture will be borne by all.
(a) This point is irrelevant to the lesson learned from the case.
(b) The characteristics of the commons are irrelevant.
(c) This point does not logically follow from the argument.
(d) This point highlights the twin aspects of individual gain and collective loss due to overuse of a common resource.
Hence, (d) is the right answer.