Which pairs of reactants will result in a chemical reaction? ?

A more reactive metal will displace a less reactive metal from the solution of its salt.

Ni(s) + MgCl₂(aq)
Ni is less reactive than Mg. Hence, no displacement occurs.

Au(s) + CoBr₂(aq)
Au is less reactive than Co. Hence, no displacement occurs.

Pb(s)+ CuNO₃(aq)
Pb is more reactive than Cu. Hence, displacement occurs.

Cr (s)+ AgClO₃(aq)
Cr is more reactive than Ag. Hence, displacement occurs.

The answers: [Pb(s)+ CuNO₃(aq)] and [Cr (s)+ AgClO₃(aq)]