The stem modifications mentioned in the options are bulb, corm, tuber, and tuber. A bulb is a type of stem modification found in onions, where the stem itself becomes fleshy and stores food. A corm is another type of stem modification found in arvi (a plant similar to taro), where the stem becomes swollen and stores food. Both a tuber of sweet potato and a tuber of potato are stem modifications where the stem becomes thickened and acts as a storage organ for food.
Therefore, the correct answer is option 3, as a tuber of sweet potato is indeed a stem modification, not a corm.
No alert is needed, as the provided answer is correct.