**Explanation:**

Frequency density is a measure used in statistics to determine the frequency of a particular class relative to its class length. It is calculated by dividing the frequency of a class by its class length.

**Class Frequency:**
- The class frequency refers to the number of observations or data points in a particular class or interval.
- In a frequency distribution table, each class or interval will have a corresponding frequency.

**Class Length:**
- The class length refers to the width or size of each class or interval in a frequency distribution table.
- It is the difference between the upper and lower class limits of a class or interval.

**Frequency Density:**
- Frequency density is calculated by dividing the class frequency by the class length.
- It gives a measure of the concentration or density of the data within a particular class or interval.

**Example:**
Let's consider a frequency distribution table with the following information:

| Class Interval | Frequency |
|---------------|-----------|
| 0-10 | 5 |
| 10-20 | 8 |
| 20-30 | 12 |
| 30-40 | 10 |

**Calculating Frequency Density:**
To calculate the frequency density, we need to divide the class frequency by the class length.

For the class interval 0-10:
- Class Frequency = 5
- Class Length = 10 - 0 = 10
- Frequency Density = 5 / 10 = 0.5

Similarly, we can calculate the frequency density for the other class intervals.

**Interpretation:**
The frequency density tells us the density or concentration of the data within each class interval. In the example above, the frequency density for the class interval 0-10 is 0.5. This means that, on average, there are 0.5 observations per unit in the interval from 0 to 10.

Therefore, option B is the correct answer as frequency density is defined as the ratio of class frequency to the class length.