The correct answer is - a) slump.
The slump is a geological occurrence that happens when a larger consolidated mass, usually composed of rocks, moves down, or falls down, a steep slope as one large mass. The movement or falling of the material is relatively short, and it can be only few ten meters, or few hundred meters. The slumps occur suddenly, usually without any warning sign, apart from the loud cracking noise few seconds before the material moves or falls down.
