The steepest angle to which a material can be piled without slumping/sliding downslope is referred to as the angle of repose.
The correct option is D.
The steepest angle of fall or dip in relation to the horizontal plane that a material may be piled to without collapsing is known as the angle of repose, or critical angle of repose, of a granular material. The materials on the slope face is almost ready to slide at this angle.
The design of particle material processing, storing, and conveying systems must take the angle of repose under consideration. The angle of repose is low when the grains are spherical and smooth. Its angle of repose is steep for extremely fine and adhesive materials.
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