Answer

The correct choice:

Red

You need to pay attention to the nose of the kettle. The blue kettle is taller than the others, but the water will be poured out through the kettle's nose. That is, the blue kettle will not be able to fill completely with water. The red kettle will be completely filled with water, because the kettle's nose is at the level with the top of the kettle. The green kettle, though it has a high nose, but it is very low.