Which of the following is a plant hormone? 

• Originally thought to be substances that, when combined with auxins, cause callus differentiation and cell proliferation, cytokinins are now understood to be much more than that. 
• The apical region of cell development and leaf withering are both aided by cytokinin. 
• Cytokinins are essential for many stages of plant growth and development. 
• They were first identified as substances that, when combined with auxins, promote cell proliferation and cause callus differentiation to shoots. 
• Throughout the plant's life, cytokinins play a crucial role in controlling growth and development.
• Insulin, Thyroxine and Oestrogen etc., are the secretions of the endocrine Glands in Human Beings.

Hence, option D is the correct answer.