Step 1: Understanding the Question:
The question asks to identify the specific plant hormone (plant growth regulator) that is responsible for and used to stimulate the formation of roots on plant cuttings.
Step 2: Key Formula or Approach:
This question requires understanding the primary functions of the main classes of plant hormones. The approach is to match the function (root initiation) with the correct hormone.
Step 3: Detailed Explanation:
Plant hormones have distinct roles in plant development.
(C) Auxin: The primary role of auxins (like Indole-3-acetic acid or IAA) is to promote cell elongation and, crucially, to initiate the formation of adventitious roots. This makes it the key ingredient in rooting powders used for vegetative propagation.
The other hormones have different main functions:
(A) Gibberellin: Primarily promotes stem elongation, germination, and flowering.
(B) Cytokinin: Primarily promotes cell division (cytokinesis) and shoot formation.
(D) Ethylene: Primarily functions as a gaseous hormone that promotes fruit ripening and senescence.
Step 4: Final Answer:
Auxins are the plant growth regulators used to induce rooting in cuttings. Therefore, option (C) is the correct answer.