Aship floating in a clear water of density 1000kg moves to sea water of density 1050kg where it floats, the upthrust on the ship then
a. stays constant
b. decreases
c. increases
d. increase by 0.05 times
e. decrease by 0.05 times

A floating object is displacing its own weight of the fluid in which it floats.
As long as the object's weight is constant, the buoyant force on it is constant.
The density of the fluid it's floating in doesn't matter.

If a mosquito lands on it, then the object weighs a tiny bit more, and it sinks
a tiny bit lower, to displace a tiny bit more fluid and achieve a tiny bit more
buoyant force.  Then it continues floating.

If a gust of wind blows some dust off of it, then the object weighs a tiny bit less,
and it floats a tiny bit higher, displacing a tiny bit less fluid, for a tiny bit less
buoyant force.  Then it continues floating.

As long as the object is floating, the density of the fluid under it is irrelevant.
The object displaces exactly as much as necessary so that the buoyant force
is exactly equal to its weight, and then it continues floating.

You KNOW that the upward buoyant force on the object MUST BE exactly equal
to the downward gravitational force on it (its weight).  If the net vertical force on it
were not zero, then the object would be accelerating vertically.  It isn't, so it is.

Strange as it may seem, I remember hearing my mother say many times:

                       "A floating object displaces its own weight.
                         A sunken object displaces its own volume."

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