The incorrect statement(s) among (a) - (c) is (are)
(a) W(VI) is more stable than Cr(VI).
(b) In the presence of HCl, permanganate titrations provide satisfactory results.
(c) Some lanthanoid oxides can be used as phosphors.

Solution

Chemistry ma’am · 4 steps

Heavier d-block metals hold high oxidation states better, and HCl itself gets oxidised by permanganate.

1. Statement (a). Down a d-block group, the higher oxidation state becomes more stable, the opposite of the p-block trend. Cr(VI) in Cr₂O₇²⁻ is a strong oxidising agent because it readily drops to Cr(III), while W(VI) in WO₃ is stable. So (a) is correct.
2. Statement (b). Permanganate is strong enough to oxidise chloride ions to chlorine gas. That side reaction uses up extra KMnO₄, so the titre comes out too high. This is why permanganate titrations are done in dilute H₂SO₄, not HCl.

   2MnO₄⁻ + 10Cl⁻ + 16H⁺ → 2Mn²⁺ + 5Cl₂ + 8H₂O

3. Statement (c). Some individual lanthanoid oxides, such as those of europium, are used as phosphors in television screens and other fluorescent surfaces. So (c) is correct.
4. So. Only statement (b) is incorrect, which is option (c).

The idea to keep. In the d-block, stability of the highest oxidation state increases down the group. And never acidify permanganate with HCl, because MnO₄⁻ oxidises Cl⁻ to Cl₂.

Answer from the NTA final answer key. Solution by PrepWiser.