What part of the caper plant is used in cooking?

The correct choice, which identifies the flower buds of the caper plant as the part used in cooking, is based on the culinary application of capers. Quercus, or capers, are the immature flower buds harvested from the caper bush (Capparis spinosa). When harvested at the right time, they are typically pickled in vinegar or brine, which enhances their unique, piquant flavor. This distinct taste makes them a popular ingredient in various cuisines, particularly in Mediterranean dishes.

The other options do not apply to culinary uses of the caper plant: the roots, leaves, and seeds of the plant are not utilized in cooking and do not possess the same culinary significance or flavor profile that the flower buds have. Therefore, the selection of the flower buds emphasizes the specific and traditional use of capers in gastronomy.