Which Apollo mission is known for demonstrating the ability to dock and transfer crew between spacecraft?

Apollo 9 is renowned for demonstrating the ability to dock and transfer crew between spacecraft, specifically between the Lunar Module and the Command Module. This mission, which took place in March 1969, was pivotal as it was the first to test the full capabilities of both spacecraft in Earth orbit.

During Apollo 9, astronauts conducted a series of critical tests, including the first-ever manned flight of the Lunar Module. This included separating the Lunar Module from the Command Module, performing a docking maneuver, and then transferring the crew from one spacecraft to the other. This was a significant achievement because successful docking was essential for future missions, especially those that would land astronauts on the Moon and then return them safely to Earth.

The other missions listed focused on different objectives. Apollo 10 served primarily as a "dress rehearsal" for the moon landing, Apollo 11 achieved the first lunar landing, and Apollo 13 was focused on a dramatic in-flight emergency, which did not involve the planned operations of docking and crew transfer that were highlighted during Apollo 9. Thus, Apollo 9 stands out for its successful demonstrations of these critical capabilities.