The spacecraft that used radar to map most of Venus's surface in the early 1990s was Magellan. Launched by NASA in 1989, Magellan arrived at Venus on August 10, 1990, and used synthetic-aperture radar to see through the planet's permanently thick cloud cover.
Visible-light cameras cannot directly survey Venus's surface from orbit because the atmosphere is opaque. Magellan instead transmitted microwave pulses toward the ground and measured their reflections. By combining observations made from slightly different positions, its radar produced detailed images of mountains, volcanoes, lava plains, channels, and tectonic structures.
The mission mapped about 98 percent of Venus's surface, creating the most comprehensive global map available for decades. Its data showed that roughly 80 percent of the planet is covered by volcanic plains and helped scientists study Venus's relatively young geological surface.
The other choices belong to missions associated with different worlds or purposes. Voyager explored the outer planets, Cassini focused on Saturn, and Mars Global Surveyor mapped Mars. Magellan is specifically associated with Venus's landmark radar survey.