The tap root is best described as:

The tap root is best described as a fleshy food storing organ because it typically develops from the primary root of the plant and grows deep into the soil. This mechanism allows the tap root to anchor the plant securely while also providing a storage medium for carbohydrates and other nutrients. The structure of a tap root is designed to support the plant's needs, particularly in times of drought or nutrient scarcity, when the plant can draw upon the stored resources to sustain itself.

In contrast, the other options highlight different types of root systems that do not align with the characteristics of a tap root. A slender, long root generally describes what could be seen in some types of fibrous roots. A shallow spreading root is typical of shallow-rooted plants and does not convey the depth or structure of a taproot. Lastly, a fibrous root system comprises many fine roots that spread out widely and do not have the fleshy, storage capability characteristic of tap roots. Therefore, the answer accurately captures the unique role and form of a tap root within plant anatomy.