A : Among all ortho halophenol , fluorophenol is least acidic .
R : Ortho - fluorophenol forms intramolecular H - bond .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false , then mark (3)
D. If both Assertion and Reason are false statements , then mark (4)

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided:

**Assertion (A):** Among all ortho halophenols, fluorophenol is least acidic.
**Reason (R):** Ortho-fluorophenol forms intramolecular hydrogen bond.

### Step-by-Step Solution:

1. **Understanding Ortho Halophenols:**
- Ortho halophenols are compounds where a hydroxyl group (-OH) and a halogen (like F, Cl, Br, I) are both attached to the benzene ring at adjacent positions (ortho position).
- The acidity of phenols is influenced by the ability of the -OH group to release a proton (H⁺ ion).

2. **Analyzing the Acidity of Fluorophenol:**
- Fluorophenol has a fluorine atom, which is highly electronegative. This electronegativity can lead to the formation of intramolecular hydrogen bonds.
- In ortho-fluorophenol, the hydrogen atom of the -OH group can form a hydrogen bond with the fluorine atom. This intramolecular hydrogen bond stabilizes the molecule and makes it more difficult for the -OH group to release a proton (H⁺).

3. **Comparing with Other Ortho Halophenols:**
- Other ortho halophenols (like ortho-chlorophenol, ortho-bromophenol, and ortho-iodophenol) do not have the same capacity for intramolecular hydrogen bonding because chlorine, bromine, and iodine are less electronegative than fluorine.
- Therefore, these compounds can more readily release a proton, making them more acidic compared to fluorophenol.

4. **Conclusion on Assertion (A):**
- Since ortho-fluorophenol is stabilized by intramolecular hydrogen bonding, it is indeed the least acidic among ortho halophenols. Thus, assertion (A) is **true**.

5. **Conclusion on Reason (R):**
- The reason states that ortho-fluorophenol forms intramolecular hydrogen bonds, which is correct. This intramolecular hydrogen bonding is responsible for the decreased acidity of fluorophenol. Thus, reason (R) is also **true**.

6. **Final Evaluation:**
- Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both A and R are true, and R is the correct explanation for A.

### Final Answer:
The correct option is **Option 1**: Both assertion and reason are true, and the reason is the correct explanation of the assertion.