Pick up the anhydrous substance from the following list ${\mathrm{Na}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}.10{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$Silica gel, Cacl₂, NaOH, NaCl

Anhydrous substances (crystalline form) Ex: $\mathrm{Nacl},{\mathrm{KNO}}_{\mathrm{3}}$
Efflorescence: Substances convert into crystal by absorbing light with loss of water.
Ex: ${\mathrm{Na}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}.10{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$
Hygroscopic: Substances can absorb moisture from the atmosphere without dissolving in it.
Ex: CaCl₂, Silica gel, CaO.