Which of the following demonstrates a double-replacement reaction?
a: na3po4 + mncl2 > mn3(po4)2 + nacl
b: caco3 > cao + co2
c: 2mg + o2 > 2mgo
d: 2agno3 + cu > 2ag + cu(no3)2

The correct answer is Option A.

Explanation:

For the given options:

Option A:  

This reaction is considered as a double displacement reaction because here exchange of ions takes place.

Option B:  

This reaction is considered as a decomposition reaction because here one substance is breaking into two smaller substances.

Option C:  

This reaction is considered as a synthesis reaction because here two smaller substances are combining in their elemental state to form a single large substance.

Option D:  

This reaction is considered as a single displacement reaction because here more reactive element displaces a less reactive element from its chemical reaction.

Hence, the correct answer is Option A.