Correct Answer is Option (B)

2Pb(NO3)2(s) → 2PbO(s) 4NO2(g) O2(g)

This reaction involves the decomposition of lead nitrate into lead oxide, nitrogen dioxide, and oxygen gas. During the reaction, lead (II) ions lose electrons and their oxidation state changes from +2 to 0, while nitrogen (V) ions gain electrons and their oxidation state changes from +5 to +4. Oxygen (0) atoms are also involved in the reaction and its oxidation state doesn't change.

Therefore, option B is the correct answer as it meets the criteria of a decomposition redox reaction.

Correct Answer is Option (C)

Metal displacement reactions - 

• It is a type of redox reaction.
• In metal displacement reaction, the most reactive metal gets oxidised easily and replaces the other less reactive metal from its compound.
• The reaction can be represented as - M₁X  +  M₂  → M₂X    (provided M₂ is more reactive than M₁)

Reactivity series - Reactivity series is a series of metals in which metals arrange according to the descending order of their reactivity. This means a high reactive metal lies in the topmost of the series.

Explanation:

Both reactions 1 and 2 are examples of decomposition reactions as a reactant is decomposed into two or more products.

Reactions 3 and 4 are examples of simple displacement reactions.

But in reaction 4, Fe metal replaces hydrogen from HCl. As 'Hydrogen' is non-metal, it is not an example of a metal displacement reaction.

So, we have left with reaction 3 only, which is an example of metal displacement reaction, because -

• In reaction 3, Al (aluminum) replaces Cr metal from its oxide. Both Al and Cr are metals.
• The reaction is possible as the position of Cr lies below than Al in the reactivity series.
• Reactivity series is a series of metals in which metals arrange according to the descending order of their reactivity.
• The position of Al in the reactivity series indicates, that Al is more reactive than Cr.
• We know that only a more reactive metal can replace less reactive metal from its oxide.
• Hence, Al can replace Cr from Cr₂O₃. 

So reaction, Cr2O3 + 2Al $\stackrel{\mathrm{\Delta }}{⟶}$ Al2O3 + 2Cr  is an example of a metal displacement reaction.

Hence, the correct answer is option 3.

Correct Answer is Option (B)

A. The reaction of $H_{2}S{O}_4$ with $NaOH$ $H_{2}S{O}_4+2NaOH\to N{a}_{2}S{O}_4+H_{2}O$ Thus its acid-base Neutralisation reaction B. In atmosphere , formation of $O_3$ from $O_2$ by lightening $3O_2\to 2O_3$ changes in one allotrope of O to another. Thus it's an allotropic formation
C. Formation of oxides of nitrogen from nitrogen and oxygen by lightening $N_2+O_2\to 2NO$ here in $N_2$ N has oxidation number 0 and in $NO$ it is +2. Thus $N_2$ is reduced and similarly, oxidation of $O$ changes from 0 to -2.and thus $O_2$ is oxidised. D. Evaporation of $H_{2}O$ $H_{2}O(l)\to H_{2}O(g)$ state changes only. Thus, the formation of oxides of nitrogen from nitrogen and oxygen by lightning is the redox reaction where oxidation, as well as reduction, takes place, simultaneously.

Correct Answer is Option (A)