The correct answer is $\boxed{\text{C}}$.
The storage capacity of the soil is the amount of water that can be held in the soil between the field capacity and the permanent wilting point. The field capacity is the amount of water that the soil can hold after it has been saturated and allowed to drain for a day or two. The permanent wilting point is the amount of water that the soil can hold after the plants have wilted and are unable to take up any more water.
The storage capacity of the soil is calculated by multiplying the depth of the root zone by the difference between the field capacity and the permanent wilting point. In this case, the depth of the root zone is 80 cm, the field capacity is 25%, and the permanent wilting point is 15%. Therefore, the storage capacity of the soil is $80 \times (0.25 – 0.15) = 12 \text{ cm}$.
Option A is incorrect because it is the depth of the root zone, not the storage capacity of the soil. Option B is incorrect because it is the field capacity, not the storage capacity of the soil. Option D is incorrect because it is the permanent wilting point, not the storage capacity of the soil.