Which of the following is NOT a plant hormone?

Step 1: Analyze the question You are asked to pick the one substance that is not a plant hormone (plant growth regulator).

Step 2: Review the concept Plant hormones are chemical messengers that regulate growth and development. The five major groups are:

• Auxins: cell elongation, apical dominance, root formation
• Gibberellins: stem elongation, seed germination
• Cytokinins: cell division, delay of leaf aging
• Abscisic acid (ABA): closes stomata, promotes dormancy, and causes leaf fall
• Ethylene: fruit ripening

Step 3: Check each option

• A. Abscisic acid: a plant hormone (growth inhibitor).
• B. Auxin: a plant hormone.
• C. Cytokinin: a plant hormone.
• D. Cytochrome: a protein with an iron-containing haem group. It carries electrons in the electron transport chain during respiration (in mitochondria) and photosynthesis (in chloroplasts). It is not a hormone.
• E. Gibberellins: plant hormones.

Step 4: Answer The substance that is not a plant hormone is Cytochrome, which is option D.

Exam tip: Plant hormones usually end in "-in" or "-ins" (auxin, cytokinin, gibberellin), but do not rely on the ending alone. Remember that cyto-chrome is a "colored cell protein" involved in respiration, not growth control. Link cytochrome with the electron transport chain and you will not confuse it with a hormone.