It is the reaction in which two compound exchange their ions and form new compounds.
AB + CD → AC +BD
So from given list of reactions option D is correct while others are incorrect.
Other examples:
The reaction between sodium chloride and silver nitrate is double displacement reaction.
NaCl + AgNO₃ → AgCl + NaNO₃
The anion and cations of both reactants exchange. The anion of sodium chloride (Cl⁻) combine with cation of silver nitrate (Ag⁺) and form AgCl. While Anion of silver nitrate combine with cation of sodium chloride and form sodium nitrate.
