A famine kills a large share of a population and within two decades the numbers have recovered. What demographic mechanism does that reveal?

D. Density relief: survivors gain land and marry earlier

Vacant holdings pull marriage earlier and earlier marriage raises completed fertility, so the population rebounds toward the ceiling that caused the crisis. It is the Malthusian system's most counter-intuitive property, the catastrophe is self-correcting demographically and changes nothing structurally, which is why the post-Black-Death wage rise eventually eroded.