Which is a cause of polarity in water molecules?
partial negative charge on the hydrogen atoms
partial positive charge on the oxygen atom
high electronegativity difference between oxygen and hydrogen
low electronegativity difference between oxygen and hydrogen

The straight answer is the third one. Usually I would go through each answer in turn before showing what the correct answer is, but it's unavoidable here. But from this truth I can show why each other one is incorrect.

Electronegativity is a relative measure of how strongly an atom attracts the shared pair of electrons in a covalent bond. The higher the electronegativity, the stronger this attraction is. When two atoms are bonded together with an electronegativity difference, assuming the difference isn't so great that ionisation occurs, a polar covalent bond exists between them where the electrons are closer to one atom than another. Here oxygen is more electronegative than hydrogen, so there is a partial positive charge on the hydrogen atom and a partial negative charge on the oxygen atom. This statement rules all the other options out!

Strictly speaking the question is poorly worded. This is not the reason water is polar. Take methane, CH4. The C-H bond is also polar, because C is more electronegative than H. However due to valence shell electron pair repulsion theory (VSEPR) we know methane has a tetrahedral shape, and in fact the symmetry of this results in the bond dipoles cancelling each other out to leave non-polarity across the molecule.

The reason water is polar is that VSEPR theory defines it as non-linear, with a bond angle of 104.5 degrees. A line of symmetry is broken, so there is an overall dipole on the molecule.

Anyway I digress. I hope this helps you :)

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