Strong heating of ferrous sulphate leads to the formation of a brown solid and two gases. This reaction can be categorised as

When green ferrous sulphate crystals are heated strongly, they undergo thermal decomposition to form a brown solid, ferric oxide ($Fe_2O_3$), and two gases, sulphur dioxide ($SO_2$) and sulphur trioxide ($SO_3$).

The balanced chemical equation is:

$$2\text{FeSO}_4(\text{s}) \xrightarrow{\text{Heat}} \text{Fe}_2\text{O}_3(\text{s}) + \text{SO}_2(\text{g}) + \text{SO}_3(\text{g})$$

• Why B is correct: This is a decomposition reaction because a single reactant breaks down into multiple simpler products. It is also a redox reaction because iron is oxidized from $+2$ in $FeSO_4$ to $+3$ in $Fe_2O_3$, while some sulphur is reduced from $+6$ in $FeSO_4$ to $+4$ in $SO_2$.
• Why A is incorrect: It is not a displacement reaction as no element is displacing another from its compound.
• Why C is incorrect: It is not a displacement reaction.
• Why D is incorrect: Although it is a decomposition reaction, it is highly endothermic (requires continuous heating to proceed), not exothermic.