The auxiliary reagent in group-IV reagent is:

Ammonium hydroxide is the auxiliary reagent found in group(iv).

Chemical auxiliaries are chemicals employed during a chemical synthesis but are not reagents since they are not integrated into the chemical product, such as solvents, separation agents, or dispersion agents.

Due to their low solubility products, group 3 cations are the only ions that precipitate as hydroxide when ${\mathrm{NH}}_4\mathrm{OH}$ is added. This is because group 3 cations have a lower solubility product than ${\mathrm{NH}}_4\mathrm{OH}$.

Hence, the option D is correct.