Identify the double displacement reaction.

2NH3 + H2SO4 ⟶ (NH4) 2SO4

2NaBr + Ca (OH) 2 ⟶ CaBr2 + 2NaOH

2NO2 ⟶ 2O2 + N2

3Pb + 2h3PO4 ⟶ 3H2 + Pb3 (PO4) 2

A double replacement reaction occurs when the ionic parts of two ionic compounds are exchanged.

Reaction 1 results in a product which is a single compound. No ionic parts are exchanged.

Reaction 3 results in one compound being separated into its elemental parts. No ionic parts are exchanged.

Reaction 4 has two reactants, but Pb is an element and not an ionic compound. No ionic parts are exchanged.

Hence, the answer is Reaction 2. Both NaBr and Ca (OH) 2 are ionic compounds. Br - is the anion in NaBr, and (OH) 2 is also an anion. Ionic parts are exchanged.