Which of the following is a type of deep foundation?

A caisson is a type of deep foundation, which is used to transfer loads from structures to deeper soil layers or bedrock. Unlike shallow foundations that are placed close to the ground surface, deep foundations like caissons are installed at a significant depth to ensure stability and support for heavy structures or when surface soil conditions are poor. They are typically drilled or dug to reach deeper soil layers, providing a reliable foundation system in scenarios where surface soils cannot adequately support the weight of the building.

The other options represent different types of foundation systems that are generally classified as shallow foundations. Crawl spaces, strip footings, and mat footings are all closer to the surface and provide support over a broader area without extending deep into the ground. Therefore, while each serves the purpose of supporting structures, only a caisson qualifies as a deep foundation due to its installation at major depths to secure the needed load-bearing characteristics.