The correct answer is overriding of pillars. Here's why:

• Overriding of Pillars: This occurs when the roof strata, particularly a massive and strong layer like sandstone, doesn't immediately cave in the goaf (the mined-out area). This results in a large overhang or cantilever. The weight and pressure from this massive overhang concentrate on the pillars closest to the goaf edge. This concentrated stress can cause the pillars to fail in compression, and the roof literally "overrides" them, pushing down and potentially causing the pillars to buckle and collapse.

Here's why the other options are less likely:

• Coal Bump: Coal bumps are sudden, violent failures of coal that can occur due to high stress concentrations. While a massive roof can contribute to high stresses, bumps are usually localized and not directly tied to crushing of pillars at the outbye (away from the working face) side due to a large goaf overhang.

• Stiffening of Pillars: Stiffening implies an increase in resistance to deformation. Pillars don't "stiffen" under excessive load; they yield or fail.

• Spalling of Pillars: Spalling is the breaking off of thin layers or fragments from the surface of the coal pillar. While spalling can occur under high stress, it's not the primary mechanism of failure when a massive roof causes pillar crushing due to a large goaf overhang. It'