Which spacecraft conducted flyby missions to investigate Mercury and Venus?

The Mariners were a series of spacecraft specifically designed for flyby missions to the inner planets, including Mercury and Venus. These missions provided crucial data about the atmospheres, surfaces, and other characteristics of these planets during the 1960s and 1970s. Mariners 10 and 2 played significant roles in gathering and relaying information about Mercury and Venus, respectively.

The other options, although significant in their respective missions, focused on different objectives and celestial bodies. The Voyagers are known for their exploration of the outer planets, while the Pioneers contributed foundational knowledge of the outer solar system, but they did not target Mercury or Venus. The Lunar Orbiters were specifically focused on the Moon, contributing to our understanding of its surface and environment. Hence, the Mariners stand out as the correct choice for investigating Mercury and Venus through flybys.