The test for $NH X_{4} X^{+}$ is always performed on a fresh portion of the original solution, not on the filtrates, because

A $NH X_{4} X^{+}$ gives no reaction with $NaOH$ after group separation
Bammonium salts are added as reagents ( $NH X_{4} Cl$ , $NH X_{4} OH$ , $(NH X_{4}) X_{2} CO X_{3}$ ) during the scheme
C $NH X_{4} X^{+}$ is precipitated in group I
D $NH X_{4} X^{+}$ is destroyed by $H X_{2} S$

Answer: B

Ammonium salts are used as group reagents in groups III to V, so

NH X_{4} X^{+}

found later would be an artefact. Heating the original solution with

NaOH

liberates

NH X_{3}

(litmus test) or gives a brown colour with Nessler reagent.