Final answer:
Cross-country mountain biking is the most popular type of competition.
Step-by-step explanation:
Out of the given options, the statement that is true regarding mountain biking is D. Cross-country mountain biking is the most popular type of competition.
Cross-country mountain biking involves riding on various terrains such as trails, forest paths, and dirt roads. It is considered one of the most popular types of mountain biking competitions due to its versatility and accessibility. Other types of mountain biking, such as downhill racing and freeride, may be more specialized and require specific conditions or equipment.