Which mission gathered information that helped Apollo planners know what to expect on the moon's surface?

The mission that provided critical information for Apollo planners regarding the lunar surface is the Surveyors. This series of robotic spacecraft were designed specifically to land on the Moon and conduct analysis of the surface. The Surveyor missions were able to send back valuable data on the Moon's soil composition, topography, and potential landing conditions, providing essential insights that directly informed the design and planning of subsequent Apollo missions.

The other missions listed do not relate to the Moon's surface exploration in the same way. The Vikings, for instance, were focused on Mars, exploring its geology and atmosphere rather than the Moon. The Rangers were primarily designed for direct impact on the lunar surface to send back images before crashing, but they were limited in analysis compared to what the Surveyors accomplished. Pioneers, on the other hand, were aimed at studying outer planets and did not contribute directly to lunar surface understanding. Thus, the Surveyor missions played a pivotal role by offering the first close-up details of the Moon, helping to ensure the success of subsequent human landings.