Which of the following environmental changes could most likely kill a large portion of a population

1. above average precipitation levels over a year

2. an annual dry season

3. cyclic temperature fluctuations

4. a flash flood

The correct answer is - 4. a flash flood.

Most of the people in the world live on the coasts or very close to the coasts in the places with very low elevation. If the circumstances are right and something manages to trigger a sudden rise in the sea level, like an asteroid/meteorite impact on the poles, than it can very easily have a devastating effect on the humans.

If an asteroid/meteorite hits one of the poles, than in only few hours the sea level will rise significantly, and that will cause a flash flood, where there will be not enough time for the people to evacuate themselves, and most of the population will be directly influenced by it, thus causing hundreds of millions, if not over a billion deaths.