Answer:
unusually cold winter
Step-by-step explanation:
Density independent factors are basically non-living things or factors that can badly influence the size of a population of living organisms. Here we are talking about the case of a flock of Canada geese on a large lake. Things that can limit their population size irrespective of members of population's physical ability or fitness is unusually cold water, because it will equally affect all members of the population and pose serious threat for their survival.
Some other examples of Density independent factors are: pollution, Earthquake , floods, temperature etc.
Hope it helps!