Decreasing order of reactivity of hydrogen halide acid in the conversion of `ROH to RX` is:

Options

A. `HCl gt HBr gt HI gt HF`
B. `HI gt HBr gt HCl gt HF`
C. `HF gt HCl gt HBr gt HI`
D. `HF gt HBr gt HI gt HCl`

Explanation

To determine the decreasing order of reactivity of hydrogen halide acids (HX) in the conversion of alcohol (ROH) to alkyl halides (RX), we need to analyze the bond strength between hydrogen and the halogen in each hydrogen halide. The reactivity of these acids is influenced by how easily the bond can dissociate.

### Step-by-Step Solution:

1. **Identify the Hydrogen Halides**: The relevant hydrogen halides are:
- Hydrofluoric acid (HF)
- Hydrochloric acid (HCl)
- Hydrobromic acid (HBr)
- Hydroiodic acid (HI)

2. **Analyze Bond Strength**: The bond strength between hydrogen and the halogen decreases as we move down the group in the periodic table from fluorine to iodine:
- HF has the strongest bond (highest bond dissociation energy).
- HCl has a weaker bond than HF.
- HBr has an even weaker bond than HCl.
- HI has the weakest bond (lowest bond dissociation energy).

3. **Reactivity Based on Bond Strength**: The weaker the bond, the easier it is for the hydrogen halide to dissociate and release H⁺ ions, which facilitates the conversion of ROH to RX:
- HI > HBr > HCl > HF

4. **Conclusion**: Therefore, the decreasing order of reactivity of hydrogen halide acids in the conversion of ROH to RX is:
- HI > HBr > HCl > HF

### Final Answer:
The decreasing order of reactivity of hydrogen halide acids in the conversion of ROH to RX is:
**HI > HBr > HCl > HF**