Question 6
Once an object is fully immersed in water, what happens to the upthrust if it is taken deeper?
The right answer is the one marked above.
The upthrust does not change. Once the object is fully immersed, the volume of water it displaces stays the same — so the weight of that water, and hence the upthrust, stays the same too.
The identical readings for cases (c) and (d) in activity 15.4 are the evidence. Pressure does rise with depth, but it rises on the upper and lower faces alike, so the difference between them is unchanged.