The correct option is D Cytokinin
Carrot root phloem assay is the bioassay for cytokinin. Cytokinins are a class of plant growth substances that promote cell division or cytokinesis in plant roots and shoots. This technique tells us about increase in cell number after cell division as it measures the growth response. This test is sensitive to amounts as low as ten- millionth of a gram of cytokinin per litre of medium.