Which US module serves as the primary laboratory on the ISS?

Destiny is the US module that serves as the primary laboratory on the International Space Station.

Also called the US Laboratory, Destiny is the main operating facility for American research payloads aboard the station. It supports experiments in fields including biology, medicine, biotechnology, materials science, physics, engineering, and Earth observation. Its racks can host a wide range of scientific equipment and sample-storage systems.

Destiny was launched aboard Space Shuttle Atlantis on mission STS-98 on 7 February 2001 and installed on the ISS three days later. Its arrival gave NASA a permanent orbital research facility for the first time since Skylab, which was vacated in 1974. The module is connected between the Unity and Harmony nodes.

The alternatives are other ISS modules with different roles. Unity is a connecting node, Kibo is Japan's laboratory, and Columbus is the European laboratory. All three support station operations or research, but Destiny is specifically the primary US laboratory.