Which of the following types of reaction occurs during the heating of iron salt when a green iron salt is heated strongly, its colour finally changes to brown, and the odour of burning sulphur is given out?

When a green iron salt is heated strongly, its colour finally changes to brown, and the odour of burning sulphur is given out. The type of reaction that takes place during the heating of iron salt is the decomposition reaction.

Ferrous sulphate is a green salt which decomposes in the presence of heat to give out three products, ferrous oxide, sulphur dioxide gas, sulphur trioxide gas. The reaction shows the breaking down of one reactant into two or more products. Therefore, it is a decomposition reaction. 

The reaction involved is-
2FeSO₄ (s) $\to$ Fe₂O₃ + SO₂ + SO₃