A displacement (single displacement) reaction occurs when a more reactive metal takes the place of a less reactive metal in that metal's salt solution. The reactivity of metals is ranked in the reactivity (activity) series — from highly reactive metals such as potassium, sodium, calcium, magnesium and zinc, down to less reactive ones like copper, silver and gold.

The essential condition is therefore that the added metal must be higher in the reactivity series than the metal present in the salt. Only then can it "push out" the other metal by giving up its electrons more readily. This makes the choice that the metal must be higher in reactivity the correct answer.

A standard example is: Zn + CuSO₄ → ZnSO₄ + Cu. Zinc lies above copper in the series, so zinc displaces copper from copper sulphate solution, and the blue colour of the solution fades while reddish copper is deposited.

The other conditions are wrong:

• The salt must in fact be soluble so that its metal ions are free in solution to be displaced; an insoluble salt would prevent the reaction, so requiring insolubility is incorrect.
• If the metal were the same as the salt's metal constituent, there would be nothing to displace — no reaction of interest occurs.
• These reactions are typically exothermic, not endothermic, so requiring an endothermic reaction is also incorrect.

If the added metal is lower in the reactivity series than the salt's metal, no displacement takes place at all.