Hydrostatics: Under which condition will a siphon fail to operate? Assume a standard siphon transferring liquid from a higher container to a lower one through a tube arching over the rim. Choose the correct option.

Concept
A siphon works because a continuous liquid column is sustained, with atmospheric pressure plus gravity maintaining flow from higher to lower level. Classic analysis uses atmospheric pressure to start and sustain the column over the crest.

Reasoning
In vacuum, the pressure support is absent and vapor cavities form, breaking the column; practical siphons fail. (Additionally, if both levels are the same or the crest exceeds barometric height for the liquid, siphoning fails.)

Final Answer
In a vacuum, where atmospheric pressure is absent