$NaH_2PO_4$ $\xrightarrow{230^\circ C}$ $Na_3P_3O_9$ $\xrightarrow{638^\circ C}$ $(NaPO_3)_n$ $\rightarrow D$ (glossy solid).
Compound $D$ is sodium hexametaphosphate,which is known as:

• A
  Bunsen’s salt
• B
  Graham’s salt
• C
  Reimann’s salt
• D
  Werner’s salt