Which of the following is not a common type of race?

The classification of races often relates to their distance and the typical participation level within running events. The 5k run, marathon, and half marathon are common race types that many people engage in, particularly in organized events and races designed for various skill levels, making them widely recognized within the running community.

On the other hand, the ultra marathon is generally defined as any race that exceeds the standard marathon distance of 26.2 miles. While ultra marathons do exist and have gained popularity among a niche group of endurance athletes, they are less common than the other distances listed. Races like the 5k, marathon, and half marathon are ubiquitous in running events and often serve as entry points for novice runners, while ultra marathons tend to attract a more specialized segment of runners who are trained for extreme endurance challenges.

Thus, identifying the ultra marathon as the least common type of race reflects its more specialized nature compared to the more frequently held and widely recognized distances of 5k, half marathon, and marathon.