Recall the composition of Nessler's reagent
Nessler's reagent is an alkaline solution of potassium tetraiodomercurate(II). It is commonly written as: $K_2[HgI_4] + KOH$ So the elements present in Nessler's reagent are:

• Potassium $(K)$
• Mercury $(Hg)$
• Iodine $(I)$
• Oxygen $(O)$ is also present because of $KOH$

Check each option

• A: Mercury — present in $[HgI_4]^{2-}$
• B: Iodine — present in $[HgI_4]^{2-}$
• C: Potassium — present as $K_2[HgI_4]$ and $KOH$
• D: Oxygen — present in $KOH$

Conclusion
All four listed elements are present in Nessler's reagent. Therefore, none of the given options is correct.

Comparison with stored answer
The stored correct answer is D (Oxygen), but this is incorrect because Nessler's reagent contains $KOH$, which includes oxygen.