Organ in the urinary system is responsible for the formation of urine

The correct answer is the kidney. If you get confused, think of kidney stones: they come out of the kidney through the urinary system.

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Many theories can never be proven. 
Here are a few examples:

-- The moon orbits the Earth.
-- Gravity pulls all masses toward each other.
-- The Earth orbits the sun.
-- Everything is made of atoms and molecules.
-- Atoms are made of protons, neutrons, and electrons.
-- Electrons moving through a wire make electric current.
-- The center of the Earth is molten iron and nickel.
-- The source of the sun's energy is nuclear fusion in its core.
-- Almost all galaxies are moving away from us.
-- The universe is expanding.

None of these statements can be proven.  That's why they're not called
Facts or Laws. 

It also tells you that some pretty powerful stuff has been done with concepts
that can't be proven.  If you want to make an idea look foolish, that doesn't
happen when you say "it's only a theory".