Interpret the sequence

We need to identify compound $A$ such that: $A \xrightarrow{\text{aqua regia}} B \xrightarrow[\text{(2) AcOH}]{\text{(1) }KNO_2/\ NH_4OH} \text{yellow ppt}$

Aqua regia dissolves many metal sulfides by oxidizing the sulfide and converting the metal into its soluble salt/chloride/nitrate form.

Meaning of the confirmatory reagent

The reagent sequence $KNO_2$ in $NH_4OH$, followed by acetic acid, is a classic confirmatory test for cobalt(II).

In ammoniacal medium, cobalt gives potassium cobaltinitrite, a yellow precipitate: $\text{Co}^{2+} \xrightarrow[K^+,\ NO_2^-]{\text{oxidation in suitable medium}} K_3[Co(NO_2)_6] \downarrow$

This yellow precipitate indicates the presence of Co.

So what is compound $B$?

Since the confirmatory test gives yellow precipitate for cobalt, $B$ must contain $\text{Co}^{2+}$ ions after dissolution in aqua regia.

Now identify $A$ from the options

The starting compounds are sulfides:

• $\mathrm{NiS}$
• $\mathrm{MnS}$
• $\mathrm{ZnS}$
• $\mathrm{CoS}$

On treatment with aqua regia, the sulfide that would produce a cobalt-containing solution is: $A = \mathrm{CoS}$

Check other options

• $\mathrm{NiS}$: would give nickel ions, not cobalt yellow ppt with $KNO_2$.
• $\mathrm{MnS}$: would give manganese ions, not this test.
• $\mathrm{ZnS}$: would give zinc ions, not this test.
• $\mathrm{CoS}$: gives cobalt ions, which produce the characteristic yellow precipitate.

Conclusion

Therefore, the compound $A$ is: $\boxed{\mathrm{CoS}}$

Hence, the correct option is D.