An underground stem that has contractile roots is
(c)Stem tuber

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Hint It stores food materials and becomes tuberous. It is not green in color and conical, cylindrical, or flattened in shape. It also bears scale leaves at each node. In the axils of these scales leaves axillary buds to arise which grow into daughter corms. The terminal bud of this is large.

Complete answer
The meanings of the following stem modifications are:
-Rhizome: It's a fleshy and horizontal stem found under the soil. Little nodes and internodes are discovered which are covered with textured leaves. Ex- Ginger, Turmeric, Canna, Water lily, and Banana.
-Corm: It is a dense structure that develops vertically under the soil surface. They have the circular node and internode and they have contractile roots. Ex- Colocasia, Alocasia, Zaminkand, etc.
-Stem tuber: The tip of the underground branches becomes swollen in the soil. Eyes are found on them which are axillary buds and that they are covered with scaly leaves. Ex- Potato.
Bulb: The stem is decreased and has a circle like structure which encompasses various plump layered leaves.
Many roots arise from its base. Food is stored in their fleshy leaves. They show apical growth. Ex- Onion and Garlic.

Additional information
The corm may be a relatively solid modified stem that features a few fleshy leaves. The node is where the leaf joins the stem. The presence of nodes and internodes makes the corm a changed stem. The papery tunic on the surface of a crocus corm is that the dead petiole from last year's leaf. Corms even have a nonpersistent rootage that lives just one season. In addition to normal fibrous roots, corms and a few non-tunicate bulbs have special roots (contractile roots). Because new corms develop on top of old ones, they're closer to the soil surface. Corms too on the brink of the surface could suffer winter injury. Contractile roots are capable of becoming deeply anchored and contracting to tug the corm deeper into the soil. Thus corms can change their depth within the soil. Example: crocus, gladiolas.

So, the correct answer is ‘Corm’.

Note: It grows into the aerial shoot and bears leaves and flowers. Adventitious roots normally develop from the bottom or everywhere the body of the corm. With the assistance of some special adventitious roots called the contractile roots or pull roots, the corm remains constantly at a specific depth. The corm propagates vegetatively by daughter corms. e.g., Amorphophallus, Colocasia, and Crocus (Saffron).