What causes a clogged floral stem due to an air embolism?

The correct answer is based on understanding how water transport works in plants, specifically through the xylem vessels. A clogged floral stem due to an air embolism occurs when a bubble of air enters the xylem. This can happen during the cutting or handling of flowers, especially if they are not placed immediately in water after being cut. The air bubble disrupts the continuous column of water that is essential for delivering nutrients and moisture from the roots to the leaves and flowers.

Once an air bubble enters the xylem, it can create a blockage, preventing water from reaching the rest of the plant tissue, which can lead to wilting and ultimately the flower's demise. Understanding this process highlights the importance of proper handling of floral stems to prevent air embolism from occurring.

The other options, while concerning for plant health or care, do not directly relate to the phenomenon of air embolism. Excessive watering, root rot, and improper stem cutting may lead to various problems in plants but would not specifically cause the indicated clogging due to air entering the vascular system.