The gases evolved on heating lead (II) nitrate crystals are:

This question is directly grounded in Activity 1.6 of the NCERT chapter, where lead nitrate powder is heated in a boiling tube. The text states: "You will observe the emission of brown fumes. These fumes are of nitrogen dioxide ($\mathrm{NO_2}$)." This is an example of thermal decomposition (Section 1.2.2), since "when a decomposition reaction is carried out by heating, it is called thermal decomposition."

The balanced equation for this reaction is:

$$2\mathrm{Pb(NO_3)_2(s)} \xrightarrow{\text{Heat}} 2\mathrm{PbO(s)} + 4\mathrm{NO_2(g)} + \mathrm{O_2(g)}$$

This is a decomposition reaction — "a single reactant breaks down to give simpler products" — where a single compound, lead(II) nitrate, decomposes on heating to give three products: lead(II) oxide (a solid residue), nitrogen dioxide (the brown fumes explicitly mentioned in the text), and oxygen gas.

Thus the two gases evolved are $\mathrm{NO_2}$ and $\mathrm{O_2}$, making option (D) correct.

Why the other options are wrong:

• (A) "NO and $\mathrm{O_2}$": Incorrect nitrogen oxide. The text specifically identifies the brown fumes as nitrogen dioxide ($\mathrm{NO_2}$), not nitric oxide (NO). NO is colourless, whereas the reddish-brown fumes described in Activity 1.6 match $\mathrm{NO_2}$.
• (B) "$\mathrm{N_2}$ and $\mathrm{NO_2}$": Incorrect — no nitrogen gas ($\mathrm{N_2}$) is produced in this decomposition; the equation shows oxygen ($\mathrm{O_2}$), not $\mathrm{N_2}$, as the second gas.
• (C) "$\mathrm{NO_2}$ and $\mathrm{H_2}$": Incorrect — lead nitrate contains no hydrogen, so hydrogen gas cannot be a product of its thermal decomposition; the second gas is oxygen, not hydrogen.