(a) W(VI) is more stable than Cr(VI). It is true and given in NCERT.
(b) in the presence of HCl permanganate titrations provide satisfactory results. It is incorrect.
Permanganate titrations in presence of hydrochloric acid are unsatisfactory since hydrochloric acid is oxidised to chlorine.
(c) Some individual Ln oxides are used as phosphors in television screens and similar fluorescing surfaces.
So,(b) is incorrect.
Therefore, the correct option is (3).
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