Selected Option: 3
Step 1: Define outbreeding mechanisms.
Outbreeding mechanisms in plants facilitate cross-pollination and increase genetic variability by inhibiting self-pollination. They ensure that pollen from one flower fertilizes the ovule of another flower, typically from a distinct plant.
Step 2: Identify xenogamy as an outbreeding mechanism.
Xenogamy signifies cross-pollination occurring between flowers of different plants. This process enhances genetic recombination and variation, which are vital for adaptation and survival in dynamic environmental conditions.
Step 3: Evaluate alternative options.
Option (A): Cleistogamy. Invalid. Cleistogamy entails self-pollination within enclosed flowers, thereby diminishing genetic diversity.
Option (B): Autogamy. Invalid. Autogamy constitutes self-pollination within the same flower, failing to promote outbreeding.
Option (D): Geitonogamy. Invalid. Geitonogamy involves pollination between flowers on the same plant, which does not lead to genuine genetic recombination.
Step 4: Conclude.
From the provided options, only xenogamy functions as an outbreeding mechanism, due to its involvement in cross-pollination between disparate plants. \[ \therefore \text{The correct answer is: Xenogamy.} \]