Question

Which reaction is a single replacement reaction?
2H2 + O2 → 2H2O
2H2O2 → 2H2O + O2
KOH + HCl → KCl + H2O
Cu + 2AgNO3 → 2Ag + Cu(NO3)2

Answer 1

usually a single replacement reaction consists of a solid being added to a solution but you did not label the states of the elements. i"m assuming it would be the last one.

Answer 2

Single replacement to me means one ion from a species is swapped with one ion from the other species,
the The Potassium Hydroxide and HydroChloric acid reaction is the only reaction mentioned like this