Among the four options, we can see gathering food, reproducing, maintaining structure, and gathering energy, so we can abord one at a time.
The statement of the question is regarding the maintaining of the population throughout the new generations, so although the answer options like "gathering food", "maintaining structure", and "gathering energy", are also important, but in the immediate survival of the species, for example. So the answer option regarding "reproducing" is better related to the statement of the question. So the answer is b, reproducing.