Question

please answer and explain this

Answer 1

C) 51g

Since the 31g object is floating, the unknown mass cannot be 28g.
By the same reasoning, the unknown mass cannot be 35g, since it would have to float higher than the 43g object.
It cannot be 80g since it would also be at the bottom of the container (since 57g object is also at the bottom)

So, by process of elimination, C) 51g is the right option.