Primary succession occurs in environments where no soil exists, such as after a volcanic eruption or glacial retreat, where the land has been exposed and lifeless for the first time.
Among the options you provided, glacial retreat (option 2) is the disturbance that would trigger primary succession, as it creates new land with bare rock and no soil, allowing new ecological communities to establish over time.
The other options (wildfire, livestock grazing, and road construction) typically involve areas that already have soil and some form of existing ecosystem, which would more likely lead to secondary succession rather than primary succession.