The correct answer is D — NO₂ and O₂.
$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\text{Heat}} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$
Heating lead(II) nitrate produces brown fumes of nitrogen dioxide and oxygen gas.
Source: Chapter 1, Section 1.2.2 (Activity 1.6)
The textbook explicitly states that brown fumes observed on heating lead nitrate are of NO₂, and the balanced equation (1.21) shows the two gases produced are NO₂ and O₂. Examiners expect you to recall this specific reaction. Note that no hydrogen is involved, ruling out options C; N₂ is not a product, ruling out B; and NO alone is not produced, ruling out A.