Explanation:Reactivity refers to the tendency of a substance to undergo a chemical reaction either by itself or with other materials and to release energy.The change of colour is a result of a chemical reaction.When A is added to the solution B changes colour because A will replace B from the solution and therefore A is more reactive than B because A tends to lose electrons and form a positive ion and change the colour of the formed ion.When C is added to the solution B does not change colour because C isnt able to replace B from the solution and therefore B is more reactive than C because C doesnt tend to lose electrons and form a positive ion and change the colour of formed ion.Therefore, from the above explanation; we can say that the correct answer is C < B < A.