Option D: 2, 2, 4, 1
The balanced equation is: $2\text{Pb(NO}_3)_2 \xrightarrow{\text{heat}} 2\text{PbO} + 4\text{NO}_2 + 1\text{O}_2$
So a = 2, b = 2, c = 4, d = 1.
Balance by checking atoms on both sides: 2 Pb(NO₃)₂ gives 2 Pb, 4 N, 12 O on LHS. RHS: 2 PbO (2 Pb, 2 O) + 4 NO₂ (4 N, 8 O) + 1 O₂ (2 O) = 2 Pb, 4 N, 12 O ✓. Students often confuse c and d; remember NO₂ gets coefficient 4, O₂ gets 1.