Which of the following pairs will not form a buffer solution? 

The buffer solutions are the mixture of weak acids and their salts or weak bases and their salts.

In option A both are salt, and hence do not form the buffer solution.

Also, the mixture of ${\mathrm{H}}_3{\mathrm{PO}}_4$ and ${\mathrm{Na}}_3{\mathrm{PO}}_4$ do not form the buffer solution.