______ is a growth inhibitor hormone in plants.

• Abscisic acid, a hormone that regulates plant growth, prevents plants from growing. It has an antagonistic effect on the hormones that promote growth, including auxin, gibberellins, and cytokinins. It promotes dormancy and aids in overcoming challenging circumstances.
• The following are some of the abscisic acid's functions in plants:
  Seed germination is inhibited as a result. Causes plant components like leaves, fruits, and flowers to abscise. Abscisic acid acts to promote dormancy in seeds and buds. RNA production is inhibited by abscisic acid. Encourages the senescence of branches and leaves. It causes the stomata to close. Plant metabolism is inhibited as a result. 
• Abscisic acid is frequently referred to as the stress hormone of plants since it boosts plants' tolerance to numerous environmental conditions, including drought and floods.

Hence, option C is the correct answer.