The aspect ratio of a rectangular duct is defined as the ratio of its width to its height. In other words, it represents the shape of the duct, specifically how elongated or squarish it is. The aspect ratio is an important parameter to consider in duct design as it directly affects the airflow characteristics and pressure drop.

The correct answer to the given question is option 'A' which states that the aspect ratio should not be greater than 8 in any case. Let's understand why this is the case.

1. Definition of Aspect Ratio:
- The aspect ratio is defined as the width (W) divided by the height (H) of the rectangular duct.
- Mathematically, Aspect Ratio = W/H.

2. Importance of Aspect Ratio:
- The aspect ratio has a significant impact on the airflow characteristics inside the duct.
- For low aspect ratios (more squarish shape), the airflow is more uniform and evenly distributed across the cross-section of the duct.
- Higher aspect ratios (elongated shape) can cause uneven airflow distribution, leading to higher pressure drop and potential flow disturbances.

3. Ideal Aspect Ratio Range:
- In general, an aspect ratio between 1 to 4 is considered desirable for rectangular ducts.
- This range ensures relatively uniform airflow distribution and minimizes pressure drop.
- Aspect ratios outside this range can lead to flow separation, increased turbulence, and pressure losses.

4. Limitation of Aspect Ratio:
- The given question states that the aspect ratio should not be greater than 8 in any case.
- This limitation is based on practical considerations and engineering experience.
- Aspect ratios greater than 8 result in highly elongated ducts, which can cause severe flow disturbances, increased pressure drop, and potential structural issues.

5. Safety and Efficiency:
- Ensuring that the aspect ratio does not exceed 8 helps maintain the safety and efficiency of the duct system.
- It ensures that the airflow remains relatively uniform, reducing the risk of flow separation and associated problems.
- Additionally, it helps minimize pressure losses, optimizing the energy efficiency of the system.

In conclusion, the aspect ratio of rectangular ducts should not be greater than 8 in any case. This limitation is based on the need for uniform airflow distribution, reduced pressure drop, and overall system efficiency.