Sweet potato is modification of.

✅ EXPLANATION:: Sweet potato is a modified adventitious root. It arises from the nodes of the stem, unlike a tap root which arises from the radicle, or a stem that grows from the nodes, or a bulb which is a modified underground stem.

📌Other Options and Explanations:
🔹 Tap root: A tap root is the main, vertical root of a plant that grows downward from the radicle.
🔹 Stem: A stem is the main above-ground axis of a plant.
🔹 Bulb: A bulb is a modified underground stem that stores food reserves.