D. All of the above.
Since the population numbers change at a predictable rate, its stable. If it was unstable, it would be an unpredictable rate and the rate would be all wacky. Same goes for the energy flow. Since the rate is constant, its stable. Because the supply of resources has a predictable rate, it's stable. Again, if it was unpredictable, it would be unstable :) hope this helps!