The equation `alpha=(D-d)/((n-1)d)` is correctly matched for: (`alpha` is the degree of dissociation, D and d are the vapour densities before and after dissociation, respectively).

Options

A.
B.
C.
D.

Explanation

To solve the problem, we need to analyze the equation given and determine which reaction corresponds to it based on the degree of dissociation (α), the vapor densities (D and d), and the number of moles of products (n).

### Step-by-Step Solution:

1. **Understand the Variables**:
- **α (degree of dissociation)**: This represents the fraction of the original substance that has dissociated into products.
- **D (vapor density before dissociation)**: This is the vapor density of the original substance before any dissociation occurs.
- **d (vapor density after dissociation)**: This is the vapor density of the system after some of the original substance has dissociated.
- **n (number of moles of products)**: This is the total number of moles of products formed after dissociation.

2. **Analyze the Equation**:
- The equation given is:
\[ \alpha = \frac{D - d}{(n - 1)d} \]
- Here, the term \(D - d\) represents the change in vapor density due to dissociation, and \((n - 1)d\) represents the effective change in moles as a result of the dissociation.

3. **Identify the Reaction**:
- We need to find a reaction where the number of moles of products (n) can be expressed correctly in relation to the degree of dissociation.
- The options provided in the question will likely represent different dissociation reactions.

4. **Evaluate Each Option**:
- For each option, calculate the total number of moles of products formed after dissociation and check if it equals \(n\).
- For example:
- If a reaction produces \( \frac{n}{2} + \frac{n}{3} \) moles, simplify it to see if it equals \(n\).
- Continue this for each provided option.

5. **Determine the Correct Option**:
- After evaluating the options, we find that the only reaction that satisfies the condition \(n = \text{total moles of products}\) is option B.

### Final Answer:
The correct option that matches the equation \( \alpha = \frac{D - d}{(n - 1)d} \) is **Option B**.