Overland flow can concentrate in small grooves called which can enlarge to form

Overland flow can concentrate in small grooves called rills, which can enlarge to form gullies. 
Between valley's at the top of interfluves water runs across the landscape in flat sheets known as overland flow.  As more and more water falls, the sheet of moving water becomes larger and larger. Eventually the water forms rills, Rills are small channels of water running across the surface of the landscape. The creation of rills happens much more quickly in areas where there is little vegetation. Many rills come together, forming larger gullies. Gullies can get quite large, and help to feed large amounts of water into streams and rivers.

The correct answer is d