Identify the reagent and the characteristic test

Dimethylglyoxime (DMG) is a very important analytical reagent used for detecting certain metal ions.

In an ammoniacal medium, DMG gives a brilliant red precipitate specifically with nickel(II) ions.

Recall the reaction

Nickel(II) reacts with dimethylglyoxime in ammoniacal solution to form a red chelate complex:

$\text{Ni}^{2+} + 2\,\text{DMG} \rightarrow \text{Ni(DMG)}_2 \downarrow$

This precipitate is characteristically bright/brilliant red.

Check the options

• A: $\text{Co}^{2+}$ — does not give the characteristic brilliant red DMG precipitate.
• B: $\text{Fe}^{2+}$ — does not give this test.
• C: $\text{Ni}^{2+}$ — yes, gives a brilliant red precipitate of nickel dimethylglyoximate.
• D: $\text{Cu}^{2+}$ — does not give this characteristic red precipitate.

Conclusion

Therefore, the metal ion is:

$\boxed{\text{Ni}^{2+}}$

So the correct option is C.