In which case would primary succession likely occur?

Primary succession occurs in environments that are newly formed or devoid of soil and life, requiring the establishment of an ecosystem from bare rock or similar substrates. The correct choice, which cites the retreat of a glacier, exemplifies this process well. As glaciers melt, they leave behind bare rock and gravel, which are initially inhospitable for most life forms. Over time, pioneer species like lichens and mosses can colonize these bare surfaces, breaking down the rock into soil and facilitating the establishment of a more complex ecosystem.

In contrast, while a forest fire can lead to secondary succession since the soil and some organisms typically remain, both floods and farming accidents are usually associated with disturbances that don't completely destroy the underlying soil or biological community. These situations usually allow for faster recovery and more immediate re-colonization of species that were already present, thus falling under secondary succession rather than primary succession.