Question 4
What happens when a hydrometer is moved from a less dense liquid into a denser one?
The right answer is the one marked above.
It sinks to a smaller depth. The hydrometer's weight is unchanged, so the weight of liquid it must displace is unchanged too — but in a denser liquid that weight occupies a smaller volume.
In a less dense liquid the same weight occupies a larger volume, so the hydrometer sinks further. It is from this depth that the scale is read.