I think the correct answer would be phloem. The vascular tissue that helps in the growth of the underground vegetable is the phloem. It is the tissue in a plant that conducts sugars and other products downward from the leaves.
Beetroot plant stores food in its root. underground tap root is modified into a bulb like structure which is used as vegetable and salad. Modified beetroot stores starch and water. therefore, tissues which conducts food and water are involved in the growth of beet root. Conducting tissue phloem which conducts food and xylem which conducts water are involved in growth of beetroot.