Which of the following items is NOT a root vegetable in the Summer Veggie Salad?

In the context of root vegetables, these are plants whose edible parts grow underground, primarily in the soil. Parsnips, turnips, and carrots are all examples of root vegetables because they grow beneath the surface and are typically harvested for their flavorful and nutritious roots.

On the other hand, red onion, while often used in salads and various dishes, is classified as a bulb vegetable rather than a root vegetable. Bulbs are a specific type of storage organ that includes layers of fleshy scales, making them distinct from the taproots and tubers that characterize root vegetables. Therefore, identifying red onion as not being a root vegetable is accurate since it belongs to a different category of vegetables used in culinary contexts.