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

The correct answer is Cytokinin

Concept:

• Phytohormones are chemical substances produced in plants that regulate various physiological processes. Key phytohormones include auxins, cytokinins, gibberellins, ethylene, and abscisic acid.
• Cytokinins are plant hormones primarily involved in promoting cell division (cytokinesis) in roots and shoots. 
• Cytokinins help overcome the apical dominance. They promote nutrient mobilisation which helps in the delay of leaf senescence.
• Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc.
• It helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.

Other Options:

• Ethylene: Ethylene is a gaseous plant hormone primarily involved in promoting fruit ripening and leaf abscission (shedding). It accelerates leaf senescence rather than delaying it.
• Abscisic Acid: Abscisic acid (ABA) is known as the "stress hormone" in plants. It plays a role in stomatal closure, seed dormancy, and stress responses. ABA generally induces senescence rather than delaying it.
• Gibberellin: Gibberellins are involved in promoting stem elongation, seed germination, and flowering.