Identify the part of the seed from the given figure which is destined to form root when the seed germinates.

The correct option is C.

Based on the provided longitudinal section (L.S.) of a monocotyledonous seed (maize grain), the labeled parts represent the following embryonic structures:
1. A (Coleoptile): The protective sheath that covers the embryonic shoot (plumule).
2. B (Plumule): The embryonic shoot that develops into the future shoot system (stems and leaves) upon germination.
3. C (Radicle): The embryonic root of the plant, which is destined to form the root system when the seed germinates.
4. D (Coleorhiza): The protective sheath that surrounds and protects the embryonic root (radicle).

Thus, the part of the seed destined to form the root is C (the radicle).