The correct option is D Undeterminable
It is not the size of the age structure but the relative size of the age structure that determines whether the population is growing, declining or stable. If the number of pre-reproductive animals are greater than the reproductive animals and post reproductive animals the population is expanding, if all the ages have equal number of animals then the population is stable, if there are more reproductive and post-reproductive animals than the pre-reproductive animals then the population is declining. Since the relative numbers are not mentioned here, the state of the population cannot be determined.