Tailing of mercury is due to the formation of:

Options

A. `Hg_2O`
B. HgO
C. `Hg(OH)_2`
D. `HgCl_2`

Explanation

To solve the question regarding the tailing of mercury, we need to analyze the options provided and understand the chemical behavior of mercury in the presence of certain agents.

### Step-by-Step Solution:

1. **Understanding Tailing of Mercury**:
- Tailing of mercury refers to the phenomenon where mercury forms a tail-like structure when it is in contact with certain materials, particularly glass. This is an important observation in various chemical processes.

**Hint**: Think about what happens to mercury when it interacts with other substances.

2. **Analyzing the Options**:
- The options given are:
- Hg2O (Mercury(I) oxide)
- HgO (Mercury(II) oxide)
- HgOH (Mercury hydroxide)
- HgStl2 (Mercury(I) sulfide)

**Hint**: Consider the oxidation states and the common oxides of mercury.

3. **Identifying the Correct Compound**:
- Among the options, Hg2O is known as mercury(I) oxide. This compound is formed when mercury is oxidized, particularly in the presence of strong oxidizing agents like ozone.
- HgO is mercury(II) oxide, which is not typically associated with the tailing phenomenon.
- HgOH and HgStl2 do not play a significant role in the oxidation process that leads to tailing.

**Hint**: Recall the role of ozone as a strong oxidizing agent and its effect on mercury.

4. **Chemical Reaction**:
- The reaction that leads to the formation of Hg2O can be represented as follows:
\[ 2 \text{Hg} + \text{O}_3 \rightarrow \text{Hg}_2\text{O} \]
- This reaction shows that when mercury is oxidized by ozone, it forms mercury(I) oxide (Hg2O), which contributes to the tailing effect.

**Hint**: Look for the reaction that explains the formation of the compound in question.

5. **Conclusion**:
- Therefore, the tailing of mercury is due to the formation of mercury(I) oxide, which is represented by the formula Hg2O.

**Final Answer**: The correct option is **Hg2O**.