Biological surplus refers to when there are more organisms than the ecosystem can sustain. Therefore, it is option A) when there are more organisms in a population than the ecosystem can sustain.
Climax community refers to when populations of animals and plants exist in balance with each other and remain stable, the final stage of succession. Therefoe, it is option D) a point in an ecosystem when all populations reach a point of little or no change.
Peak phenomena relates to booms in population when food is available and and busts when it run out. Therefore, it is option C) a population grows at a fast rate, then quickly drops when the resources run out.
Sigmoid phenomena relates to a rapid increase in population due to food and habitat availability, and a decrease due to regulatory factors (such as lower birth rate and reduced food availability). Therefore it is option B) a population increases while food and habitat are abundant and decreases as food becomes less available.