Which substance would be most suitable for row 3? a sodium chloride b silver c nitrogen d propane

The substance that would be most suitable for row 3 is SODIUM CHLORIDE.
The periodic table is made up of groups and periods, the periods are also called rows. There are seven rows and 18 groups in the periodic table. The third period is made up of 8 elements; both sodium and chlorine are part of the eight elements, thus, sodium chloride is more likely to be found on the third row. 

They are all decomposition reactions.