Questions: When a cold air mass catches up to a warm air mass, it's called a(n) front.

The answer is C: Occluded.

Explanation for each option: A. Cold - A cold front occurs when a cold air mass moves into an area occupied by a warmer air mass, but it does not describe the situation where a cold air mass catches up to a warm air mass. B. Stationary - A stationary front occurs when two air masses, one warm and one cold, meet and neither has the strength to move the other, resulting in a standstill. C. Occluded - An occluded front happens when a cold air mass catches up to a warm air mass, lifting the warm air off the ground and often leading to complex weather patterns. D. Warm - A warm front occurs when a warm air mass moves into an area occupied by a colder air mass, but it does not describe the situation where a cold air mass catches up to a warm air mass.

Summary: When a cold air mass catches up to a warm air mass, it is called an occluded front.