Which spacecraft tested soil samples on Mars?

The Viking program, which includes both Viking 1 and Viking 2 spacecraft, is well known for its successful missions to Mars in the 1970s. These spacecraft were the first to perform extensive scientific experiments on the Martian surface, specifically designed to analyze soil samples for evidence of life and to understand the planet's climate and geology. The Viking landers were equipped with instruments that could analyze chemical components of the Martian soil, conduct atmospheric experiments, and capture images of the surface, making significant contributions to our understanding of Mars.

The other spacecraft listed, while part of important space exploration missions, did not focus on soil sampling on Mars. The Pioneers primarily conducted flybys of the outer planets, the Mariners focused on significant missions to Venus and Mars but did not sample Martian soil, and the Surveyors were lunar landers that landed on the Moon to analyze its surface. Therefore, the Viking missions stand out as the correct answer for having tested soil samples on Mars.