What was the first US manned space flight?

The first US manned space flight was under the Mercury program, which marked a significant milestone in the history of space exploration. Launched on May 5, 1961, astronaut Alan Shepard became the first American to travel into space aboard the Mercury-Redstone 3 spacecraft, famously known as Freedom 7. This flight lasted about 15 minutes, during which Shepard reached an altitude of 116.5 miles, experiencing a brief period of weightlessness before safely returning to Earth.

The Mercury program focused on developing the technology and experience necessary for human spaceflight. It laid the groundwork for subsequent programs, including Gemini and Apollo, which further advanced the capabilities of manned space missions. Other options like Gemini and Apollo came later, with Gemini working on longer duration flights and Lunar missions, while Apollo was known for landing the first humans on the Moon. The Soyuz program is a Russian initiative and not part of the early US space endeavors, thus not applicable in this context. Therefore, Mercury's historical significance as the first manned space flight program makes it the correct answer.