$Y$ in the given sequence of reactions is

$$ \begin{gathered} \mathrm{P}_4+x \mathrm{NaOH}+y \mathrm{H}_2 \mathrm{O} \xrightarrow{\mathrm{CO}_2} X+z \mathrm{NaH}_2 \mathrm{PO}_2 \\ X+\mathrm{CuSO}_4 \longrightarrow Y+\mathrm{H}_2 \mathrm{SO}_4 \end{gathered} $$

Identify the element for which +1 oxidation state is more stable than +3 oxidation state.

Identify the correct statements from the following.

(i) Ti (IV) is more stable than Ti (III) and Ti (II).

(ii) Among $3 d$-series elements (From $Z=22$ to 29). Only copper has positive reduction potential $\left(M^{2+} / M\right)$.

(iii) Both Sc and Zn exhibit +1 oxidation state.

The product formed during thermal decomposition of ammonium dichromate are