Which element is not present in Nessler's reagent?

Nessler’s reagent is widely used for the qualitative detection of $$NH_3$$ (ammonia) in laboratory analysis.

Definition of Nessler’s reagent:
It is a strongly alkaline aqueous solution of potassium tetraiodomercurate(II), whose principal formula is written as $$K_2[HgI_4]$$ (sometimes shown with excess $$KOH$$ added separately to maintain high alkalinity).

Elements present in the complex ion $$[HgI_4]^{2-}$$ and the accompanying cation $$K^+$$:
• $$Hg$$   : Mercury
• $$I$$      : Iodine
• $$K$$   : Potassium

The complex itself, $$K_2[HgI_4]$$, contains only these three elements. Oxygen does not appear in the formula of the complex, and its presence is not essential for the functioning of the reagent (any $$KOH$$ that may be used for alkalinity does not enter the stoichiometric formula of the active reagent).

Conclusion:
Among the given options, the element that is not a constituent of Nessler’s reagent is $$\textbf{Oxygen}$$.

Hence, the correct option is Option D.