Assertion (A): HBr is weaker acid than HI.
Reason (R ): HBr is more polar than HI.

Explanation

To analyze the assertion and reason given in the question, we will break down the concepts of acidity, bond length, bond energy, and polarity of hydrogen halides step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that HBr is a weaker acid than HI.
- To evaluate this, we need to consider the acidity of hydrogen halides (HF, HCl, HBr, HI) and their ability to donate protons (H⁺ ions).

2. **Acidity of Hydrogen Halides**:
- The order of acidity for hydrogen halides is: HI > HBr > HCl > HF.
- This means that HI is a stronger acid than HBr. Therefore, the assertion that HBr is weaker than HI is **correct**.

3. **Understanding the Reason**:
- The reason states that HBr is more polar than HI.
- Polarity in hydrogen halides is influenced by the electronegativity of the halogen atoms.
- Fluorine (F) is the most electronegative, followed by chlorine (Cl), bromine (Br), and iodine (I).

4. **Polarity of HBr and HI**:
- HBr is more polar than HI because bromine is more electronegative than iodine.
- Therefore, the reason is also **correct**.

5. **Evaluating the Relationship**:
- While both the assertion and reason are correct, the reason does not adequately explain the assertion.
- The strength of an acid is more closely related to bond length and bond energy rather than polarity alone.
- HI has a longer bond length than HBr, which results in lower bond energy, allowing HI to dissociate more easily and thus be a stronger acid.

6. **Conclusion**:
- Both the assertion (A) and the reason (R) are correct, but the reason is not the correct explanation for the assertion.

### Final Answer:
- Assertion (A) is correct: HBr is weaker than HI.
- Reason (R) is correct: HBr is more polar than HI.
- However, the reason does not explain the assertion.