Concept:Thinning out is the removal of excess seedlings from a stand so that the remaining ones grow well.Explanation:When too many seedlings come up in one stand, they compete for light, water, nutrients and space. Removing the weak extra ones gives the rest room to grow.Earthing up is heaping soil around the base of plants. Filling in (supplying) is planting in empty stands where seeds failed. Pricking out is moving young seedlings from a nursery box to a nursery bed.Answer:Option D (thinning out)