Assertion : Aluminium and bromine form `AlBr_3` as a stable binary compound.
Reason : Aluminium is group 13 element while bromine is a group 17 element.

Options

A. Both A and R are true and R is correct explanation of the assertion
B. Both A and R are true and R is not correct explanation of the assertion
C. A is true but R is false
D. A is false but r is true

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that Aluminium and Bromine form `AlBr_3` as a stable binary compound.
- This is true. Aluminium (Al) reacts with Bromine (Br) to form Aluminium Bromide, which has the chemical formula `AlBr_3`.

2. **Understanding the Reason**:
- The reason given is that Aluminium is a group 13 element while Bromine is a group 17 element.
- This is also true. Aluminium belongs to Group 13 of the periodic table, and Bromine belongs to Group 17.

3. **Valence of Elements**:
- Aluminium has a valence of +3 (it can lose three electrons).
- Bromine has a valence of -1 (it can gain one electron).

4. **Formation of the Compound**:
- To form a stable compound, the total positive charge must balance the total negative charge.
- In `AlBr_3`, one Aluminium ion (Al^3+) combines with three Bromine ions (3 Br^-), resulting in a neutral compound.

5. **Conclusion**:
- While both the assertion and reason are true, the reason does not adequately explain why `AlBr_3` is formed as a stable binary compound. It merely states the group numbers of the elements without detailing the charge balance that leads to the formation of the compound.

### Final Answer:
- The assertion is true, and the reason is true, but the reason is not the correct explanation for the assertion.