The decreasing order of acidic character of the following is :
(I) `CH_3 SH`
(II) `CH_3 OH`
(III) `H_2 O`
(IV) `EtOH`.

Options

A. `I gt II gt III gt IV`
B. `IV gt III gt II gt I`
C. `I gt III gt II gt IV`
D. `III gt I gt II gt IV`

Explanation

To determine the decreasing order of acidic character among the compounds CH₃SH (methanethiol), CH₃OH (methanol), H₂O (water), and EtOH (ethanol), we can analyze the acidity based on their ability to donate protons (H⁺ ions).

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (I) CH₃SH (methanethiol)
- (II) CH₃OH (methanol)
- (III) H₂O (water)
- (IV) EtOH (ethanol)

2. **Understand Acidity**:
- The strength of an acid is often measured by its pKa value; the lower the pKa, the stronger the acid.
- Acids donate protons (H⁺), and the stability of the resulting conjugate base is a key factor in determining acidity.

3. **Compare Thiols and Alcohols**:
- Thiols (R-SH) are generally more acidic than alcohols (R-OH) because sulfur is larger than oxygen, leading to a longer and weaker S-H bond compared to the O-H bond. This makes it easier for thiols to lose a proton.
- Therefore, CH₃SH is more acidic than CH₃OH.

4. **Compare Alcohols and Water**:
- Methanol (CH₃OH) has a pKa of approximately 15.5, while water (H₂O) has a pKa of about 15.7. Since 15.5 < 15.7, methanol is more acidic than water.
- Ethanol (EtOH) is also an alcohol and has a similar structure to methanol, but it is slightly less acidic than methanol due to the presence of an ethyl group which has a +I (inductive) effect that increases electron density, making it less willing to donate a proton.

5. **Final Comparison**:
- Based on the above analysis, we can summarize the order of acidity:
- CH₃SH > CH₃OH > H₂O > EtOH

### Conclusion:
The decreasing order of acidic character is:
**CH₃SH > CH₃OH > H₂O > EtOH**