Q.In which of the following + 2 oxidation state is the most stable ?

The +2 state is most stable for Pb (inert pair effect).

Down group 14, the ns2 electrons become increasingly reluctant to take part in bonding — the inert pair effect. As a result the lower (+2) oxidation state becomes more stable relative to +4 as we descend the group. So for lead …

Lead (Pb) shows the inert pair effect, making Pb2+ more stable than Pb4+.

The inert pair effect refers to the tendency of the outermost s-electron pair (ns2) to resist participating in bond formation, especially for the heavier elements of groups 13-15. This becomes more pronounced going down a group due to poor shielding by intervening d/f electrons and relativistic effects, which stabilise the s-electrons closer to the nucleus.

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Ammonia + Nessler's reagent → brown precipitate — option (d).

Nessler's reagent is an alkaline solution of potassium tetraiodomercurate(II), K₂[HgI₄]. With ammonia it forms a brown precipitate (basic mercury(II) amido-iodide, the iodide of Millon's base): …

Nitric acid (Ostwald process) uses a platinum catalyst — option (d).

In the Ostwald process, ammonia is oxidised over a hot platinum–rhodium gauze:

4NH₃ + 5O₂ →(Pt, ~500 °C) 4NO + 6H₂O

2NO + O₂ → 2NO₂ …

Heating ammonium dichromate releases nitrogen gas — option (b).

From NCERT Class 11 Chemistry (nitrogen chemistry / redox): ammonium dichromate undergoes an internal redox decomposition on heating:

(NH₄)₂Cr₂O₇ → N₂↑ + Cr₂O₃ + 4H₂O. …