Stem
The stem of a plant is the part that provides support to the upper part of the plant. It is the main axis of the plant, connecting the roots to the leaves, flowers, and fruits. The stem plays a crucial role in transporting water, minerals, and nutrients between the roots and the upper parts of the plant. It also provides structural support to keep the plant upright and allows for the efficient distribution of resources.

Functions of the Stem:
The stem performs several important functions in a plant's life cycle:

- Support: The stem provides structural support to the plant, keeping it upright and allowing it to grow vertically towards the sunlight. Without a strong stem, the plant would not be able to stand upright and would collapse under its own weight.

- Transport: The stem contains specialized tissues called xylem and phloem, which are responsible for transporting water, minerals, and nutrients throughout the plant. The xylem transports water and minerals from the roots to the leaves, while the phloem transports sugars and other organic compounds from the leaves to other parts of the plant.

- Storage: Some plants have stems that are adapted to store water or nutrients. For example, cacti have thick, fleshy stems that store water to survive in arid environments. Other plants store starches and sugars in their stems as a reserve for future use.

- Vegetative Propagation: Some plants can reproduce asexually by producing new individuals from their stems. This is commonly seen in plants like strawberries, where runners or stolons emerge from the stem and produce new plantlets.

Conclusion:
In summary, the stem of a plant provides support to the upper part of the plant, allowing it to grow vertically. It also plays a crucial role in transporting water, minerals, and nutrients, storing resources, and facilitating vegetative propagation. Without a strong and healthy stem, a plant would not be able to survive and thrive. Therefore, the correct answer to the question is option 'C' - stem.