The reaction between the iron nail and copper sulphate solution is an example of a displacement reaction. In a displacement reaction, one element displaces another element from its compound.

Option 1: Combination - This type of reaction occurs when two or more substances combine to form a new substance. It does not apply to the reaction between iron nail and copper sulphate solution.

Option 2: Decomposition - This type of reaction occurs when a compound breaks down into two or more simpler substances. It does not apply to the reaction between iron nail and copper sulphate solution.

Option 3: Displacement - This is the correct answer. In this reaction, the iron in the nail displaces copper from the copper sulphate solution, forming iron sulphate and depositing copper on the surface of the nail. This displacement reaction causes the color change of the nail to brownish and the fading of the blue color of copper sulphate solution.

Option 4: Double displacement - This type of reaction occurs when the positive and negative ions of two compounds switch places to form two new compounds. It does not apply to the reaction between iron nail and copper sulphate solution.

In conclusion, the reaction between the iron nail and copper sulphate solution is an example of a displacement reaction.