Which one of the following phytohormones promotes nutrient mobilization which helps in the delay of leaf senescence in plants?

The correct option is Cytokinin.

Step-by-Step Explanation:

1. Understanding Leaf Senescence: Leaf senescence is a genetically programmed, active developmental process in plants characterized by the degradation of cellular components, chlorophyll breakdown, and eventual death of the leaf tissues. During this phase, nutrients from older leaves are typically remobilized to younger tissues or storage organs.

2. Role of Cytokinins: Cytokinins are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. One of their most well-documented physiological functions is their ability to delay leaf senescence, a phenomenon often referred to as the Richmond-Lang effect.

3. Mechanism of Nutrient Mobilization: Cytokinins delay senescence primarily by creating a metabolic sink. They stimulate the mobilization of nutrients, amino acids, and other resources to the site of cytokinin application or high concentration. This active nutrient mobilization ensures that the leaf maintains its metabolic activity, retains chlorophyll, and continues photosynthesis, thereby delaying the onset of senescence.

4. Comparing Other Phytohormones:
- Ethylene: Promotes fruit ripening and hastens leaf senescence and abscission.
- Abscisic acid (ABA): Known as a stress hormone, it actively induces senescence and dormancy, and promotes stomatal closure.
- Gibberellin: Mainly involved in stem elongation, seed germination, and flowering, rather than specific nutrient mobilization-led senescence delay in this context.

Therefore, Cytokinin is the specific phytohormone responsible for promoting nutrient mobilization, which helps in the delay of leaf senescence.