Answer
To tip the object, it must be tilted until its CG no longer is above its footprint (support base).
An object is in stable equilibrium when it requires positive work to be done on it, producing an increase in PE before it can topple. (In contrast, a cone balanced on its tip will lower its PE as it tilts, making it a situation of unstable equilibrium.)
The diagram shows that the narrow cylinder undergoes the least change in PE (relative to its own mass) before it reaches the point of no return. It is the least stable. The third object requires the most work to reach the tipping point, and is the most stable.
This is discussed on page 144.
