**Explanation:**

The plasticity index (PI) of a soil is a measure of its plasticity, which refers to the ability of a soil to change shape and deform without breaking. It is an important property of soils, especially in engineering applications such as foundation design and slope stability analysis.

The plasticity index is determined by conducting a standardized laboratory test called the Atterberg limits test. The test involves determining the moisture content at which a soil transitions from a plastic state to a liquid state (liquid limit) and from a plastic state to a semi-solid state (plastic limit). The plasticity index is then calculated by subtracting the plastic limit from the liquid limit.

Now, let's analyze the given scenario:

- The plasticity index of the soil is 45%.

To understand what this means, we need to consider the range of plasticity index values:

- Soils with a plasticity index of less than 7% are classified as non-plastic.
- Soils with a plasticity index between 7% and 17% are classified as low plastic.
- Soils with a plasticity index between 17% and 35% are classified as medium plastic.
- Soils with a plasticity index greater than 35% are classified as highly plastic.

In this case, the plasticity index of 45% falls within the range of highly plastic soils. This means that the soil has a high degree of plasticity, indicating that it can be easily molded and deformed without breaking. Highly plastic soils typically have high clay content, which gives them their plastic properties.

Therefore, the correct answer is option B: very highly plastic.

In summary, the plasticity index of a soil is a measure of its plasticity, and a value of 45% indicates that the soil is highly plastic, meaning it can be easily molded and deformed without breaking.

B