Application of ________ can delay abscission of fruits and leaves at early stages

The correct option is auxin.

Understanding Abscission:
Abscission is the physiological process by which plants shed their parts, such as leaves, flowers, and fruits. This process is highly regulated by plant hormones (phytohormones) and occurs at a specific region called the abscission zone, which is located at the base of the petiole or pedicel.

Role of Auxin in Delaying Abscission:
Auxins play a critical role in controlling the timing of abscission. Their effect depends on the developmental stage of the plant organ:
1. Early Stages: In young, actively growing leaves and fruits, the concentration of auxin is naturally high. Auxin acts as an inhibitor of abscission by preventing the synthesis and activity of cell-wall degrading enzymes (such as cellulase and pectinase) in the abscission zone. Consequently, applying exogenous auxin at these early stages helps to maintain high hormone levels, thereby delaying premature leaf fall and fruit drop.
2. Mature/Late Stages: As the leaf or fruit matures and ages (senesces), the production of auxin decreases significantly. This drop in auxin levels makes the abscission zone highly sensitive to ethylene, another plant hormone that promotes cell wall breakdown and leads to shedding.

Why other options are incorrect:
- Ethylene: This hormone is a gaseous plant regulator that actively promotes abscission and ripening rather than delaying it.
- Gibberellic Acid and Cytokinin: While these hormones are involved in cell growth, division, and delaying senescence, they do not play the primary direct role that auxin does in preventing abscission at the early developmental stages of leaves and fruits.