Answer: All four!
Every year in the US, there are about 700,000 deaths due to heart disease; 500,000 from cancer; 160,000 caused by strokes; and 110,000 from lung diseases.[i] Active exercise like cycling helps reduce all these, not to mention 40,000 motor vehicle fatalities.
Now stop and think about those numbers! Compare those with the number of bicycling fatalities. In a typical year in America, there are less than 800 cycling fatalities. There were only 690 in the year 2000. For every cycling death, there are roughly 1000 deaths due to heart disease - and cycling can help prevent heart disease!
[i]. World Almanac 2000, Primedia Reference Inc.