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.

We begin by examining statement (a). Chromium, molybdenum and tungsten all belong to Group 6. A well-known periodic trend says that higher oxidation states become more stable as we move down a group. The reason is that the heavier atoms have larger, more diffuse d-orbitals and a higher effective nuclear charge, so they can better accommodate loss of more electrons. Hence the $$+6$$ oxidation state of tungsten, $$\text{W(VI)},$$ is expected to be more stable than the $$+6$$ state of chromium, $$\text{Cr(VI)}.$$ So statement (a) is correct, and therefore it is not an incorrect statement.

Next we consider statement (b). In a typical redox titration using permanganate $$\left(\mathrm{MnO_4^-}\right),$$ we want only the principal reaction

$$\mathrm{MnO_4^- + 8\,H^+ + 5\,e^- \;\longrightarrow\; Mn^{2+} + 4\,H_2O}$$

to occur. For that reason the titration is carried out in a moderately strong but non-reducing acid, most commonly dilute $$\mathrm{H_2SO_4}$$. If we use dilute $$\mathrm{HCl}$$ instead, the chloride ions act as a reducing agent and side reactions take place, e.g.

$$\mathrm{2\,Cl^- \;\longrightarrow\; Cl_2 + 2\,e^-}$$

and simultaneously

$$\mathrm{MnO_4^- + 8\,H^+ + 5\,e^- \;\longrightarrow\; Mn^{2+} + 4\,H_2O}.$$

The liberated $$\mathrm{Cl_2}$$ not only consumes MnO$$_4^-$$ but also escapes as a gas, giving an indeterminate end point. Because these side reactions spoil the accuracy of the titration, we say that permanganate titrations in the presence of $$\mathrm{HCl}$$ do not give satisfactory results. Thus statement (b) is incorrect.

Finally we look at statement (c). Certain oxides of lanthanoids (also called lanthanides) such as $$\mathrm{Y_2O_3:Eu^{3+}}, \; \mathrm{LaPO_4:Ce^{3+},Tb^{3+}}$$ and $$\mathrm{Y_2O_3:Tb^{3+},Eu^{3+}}$$ emit visible light when excited by ultraviolet or cathode-ray radiation. These materials are widely used as phosphors in fluorescent lamps, television and computer display screens. Because such practical applications exist, statement (c) is correct, and hence it is not an incorrect statement.

Collecting our results, we observe that only statement (b) is wrong. Therefore the list of incorrect statements is simply $$\{(b)\}.$$ So, among the options provided, the choice that mentions (b) only is Option C.

Hence, the correct answer is Option C.