NH₃ gas is dried over :

Ammonia is basic and Quick lime (CaO) is also basic, therefore can be used for drying ammonia. 

Nitric acid, sulfuric acid and phosphorus pentoxide are acidic in nature and ammonia is basic. Therefore, when sulfuric acid or phosphorus pentoxide or nitric acid is treated with ammonia, then an acid-base reaction takes place. Therefore, sulfuric acid, nitric acid and phosphorus pentoxide cannot be used as drying agents for ammonia.

Sulfuric acid forms ammonium sulfate when reacts with ammonia.

$2{\mathrm{NH}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$

Phosphorus pentoxide forms ammonium phosphate when it reacts with ammonia.

$6{\mathrm{NH}}_3+{\mathrm{P}}_2{\mathrm{O}}_5+3{\mathrm{H}}_2\mathrm{O}\to {2\left({\mathrm{NH}}_4\right)}_2{\mathrm{PO}}_4$

Nitric acids forms ammonium nitrate when it reacts with ammonia.

${\mathrm{NH}}_3+{\mathrm{HNO}}_3\to {\mathrm{NH}}_4{\mathrm{NO}}_3$