(d) : Both the populations are herbivorous, thus they cannot affect each other. If the food sources for these populations $A$ and $B$ have decreased, then both the populations $A$ and $B$ would have declined. If population $A$ have produced more offspring then the graph $A$ should have increased. Based on the graph, population $B$ is more successful in competing with population $A$, that is why number of organisms in population $B$ increased while that in population $A$ decreased, as they get access to limited resources.