Final answer:
The steepest angle of repose is associated with angular and dry grains, as their shape contributes to higher friction which stabilizes the pile.
Step-by-step explanation:
The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is determined by the friction between the grains. When considering the types of grains that would have the steepest angle of repose, angular and dry grains are more likely to maintain a steeper angle because they have more friction resisting movement compared to rounded or wet grains. Wet grains can lead to a decrease in the angle of repose due to water acting as a lubricant between particles, while rounded grains have less friction because their smooth surfaces slide past each other more easily. Therefore, angular and dry grains are the correct answer for the type of grains that will have the steepest angle of repose when piled.