Assertion (A) : Among the hydrogen halides HF is weakest acid.
Reason R: Among hydrogen halides HF bond length is least.

Options

A. A & R are true, R is correct explanation of A
B. A & R are true, R is not correct explanation of A
C. A is true, but R is false
D. A is false, but R is true

Explanation

To solve the question regarding the assertion and reason about hydrogen halides, we will analyze both statements step by step.

### Step-by-step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that among the hydrogen halides (HF, HCl, HBr, HI), HF is the weakest acid.
- To evaluate this, we need to consider the strength of acids in terms of their ability to donate protons (H⁺ ions).

2. **Understanding the Reason (R)**:
- The reason states that among hydrogen halides, HF has the least bond length.
- Bond length is the distance between the nuclei of the two atoms involved in the bond. A shorter bond length typically indicates a stronger bond.

3. **Analyzing the Bond Strength**:
- In hydrogen halides, as we move down the group from fluorine to iodine, the size of the halogen atom increases. This results in longer bond lengths (H-F < H-Cl < H-Br < H-I).
- A longer bond length usually means a weaker bond because the shared electrons are farther apart from the nuclei, making it easier for the bond to break.

4. **Evaluating Acidity**:
- The strength of an acid is related to how easily it can lose a proton (H⁺).
- HF, with a shorter bond length and stronger bond, does not release H⁺ ions as easily as the other hydrogen halides (HCl, HBr, HI), which have longer and weaker bonds.
- Therefore, HF is indeed the weakest acid among the hydrogen halides.

5. **Conclusion on Assertion and Reason**:
- Both the assertion (A) and the reason (R) are true. However, the reason does not correctly explain why HF is the weakest acid. The weak acidity of HF is due to its strong H-F bond, not just because it has the least bond length.
- Thus, while both statements are true, the reason does not justify the assertion correctly.

### Final Answer:
- The assertion (A) is true, and the reason (R) is true, but R does not explain A correctly. Therefore, the correct conclusion is that both statements are true, but the reason is not a valid explanation for the assertion.