Which of the following is a type of deep foundation?

Deep foundations are structural elements that transfer building loads down to deeper soil layers that are more capable of bearing these loads. They are typically used in situations where surface soils do not have adequate strength or stability. A caisson is a specific type of deep foundation that is constructed by drilling a shaft into the ground and filling it with concrete to support heavy loads, making it suitable for use in various soil conditions and depths.

The other options listed—crawl spaces, strip footings, and mat footings—are generally associated with shallow foundations. A crawl space typically involves a raised floor structure above the ground that allows for ventilation but does not extend into deeper ground layers like a caisson does. Strip footings are continuous strips of concrete that support walls and are placed at shallow depths. Mat footings cover a large area and provide support to buildings by distributing load over a wide base, but they also do not reach significant depths like caissons. Thus, caisson stands out as the correct answer since it is explicitly designed for deeper applications.