If Earth's surface cooled but water vapour was unavailable, which condition for the hydrosphere would remain incomplete?

Correct answer: A water source

The governing concept is that formation of the hydrosphere required both a sufficiently cooled surface and an available supply of water. Cooling would allow liquid water to exist rather than remaining entirely as vapour, but cooling alone cannot create water. If water vapour were absent, there would be no material to condense, fall as rain, or collect in depressions and basins. Thus option B is correct: the water source would remain missing. A stable surface might provide places for water to accumulate, and gravity would help water move and collect, but neither supplies water. The metallic core is important to Earth’s structure but is not the missing condition in this situation.