The chemical reactions involved in all the four tubes are listed.
`"Test tube A : "underset("(Lightgreen)")(3FeSO_(4)(aq))+2Al(s)rarrunderset("(Colourless)")(Al_(2)(SO_(4))_(3)(aq))+3Fe(s)`
`"Test tube B : "underset("(Blue)")(3CuSO_(4)(aq))+2Al(s)rarrunderset("(Colourless)")(Al_(2)(SO_(4))_(3)(aq))+3Cu(s)`
`"Test tube C : "underset("(Colouless)")(Al_(2)(SO_(4))_(3)(aq))+Fe(s)rarrunderset("(No colour change)")("No chemical reaction")`
`"Test tube D : "underset("(Blue)")(Cu(SO_(4))(aq))+Fe(s)rarrunderset("(No colour change)")(FeSO_(4)(s)+Cu(s))`
Change of colour will be observed in test tubes A, B, and D while the solution will remain colourless in test tube C.
On the basis of the chemical reactions, listed above, Aluminium (Al) is the most reactive metal because it has displaced all other metals from their aqueous salt solutions.