This is a redox reaction because the oxidation state of hydrogen changes from 0 to +1, and the oxidation state of chlorine changes from 0 to -1.

B: CaCO3(s) → CaO(s) + CO2(g)

This is also a redox reaction because the oxidation state of carbon changes from +4 in CaCO3 to +2 in CO2.

C: 2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)

This is a redox reaction because the oxidation state of copper changes from 0 to +2, and the oxidation state of silver changes from +1 to 0.

D: NH3(g) + HCl(g) → NH4Cl(s)

This is not a redox reaction because the oxidation states of all the atoms remain the same throughout the reaction.