Which reaction is single replacement?
(1) 2h2o2 --> 2h2o + o2
(2) 2h2 + o2 --> 2h2o
(3) h2so4 + mg --> h2 + mgso4
(4) hcl + koh --> kcl + h2o

Among the given choices, the third option is the only one which illustrates single replacement. (3)H2SO4 + Mg --> H2 + MgSO4 A single replacement is also termed as single-displacement reaction, a reaction by which an element in a compound, displaces another element. It can be illustrated this way: X + Y-Z → X-Z + Y

In a single replacement reaction (or simple displacement) an element reacts with a compound and takes the place of another element in that compound. The reaction that shows a sigle replacement is the third one. Where H2SO4 and Mg turn into H2 + MgSO4. In this case Mg replaces H2