Which type of erosion can place a boulder in the middle of a field?

Answer: Plucking.

Explanation:

Plucking is a glacial phenomenon that cuases erosion and it moves individual pieces of bedrock. When a glacier moves down a valley, the friction melts the basal ice, which then infiltrates cracks in the bedrock. As the glacier flows, mechanical weathering releases rock on the valley walls, which drops as debris on the glacier. Glaciers can carry both giant boulders and silt.

Probably a pluvial erosion, where it is caused by the rains causing deep wear of the soils without vegetation. More precisely the erosion in splash, which is the effect generated by the impact of raindrops on the ground. Apparently simple, this process can cause greater problems if it is intensified by the total or partial disaggregation of the soil particles and the rocks, causing them to roll.