The correct answer is option 'A' (IBA).

IBA, or Indole-3-butyric acid, is a type of plant hormone known as an auxin. Auxins play a crucial role in various plant growth and development processes, including root initiation.

Here is a detailed explanation:

1. Role of auxins in plant growth and development:
- Auxins are a class of plant hormones that regulate various physiological and developmental processes in plants.
- They are mainly produced in the apical meristems of plants and are transported to different parts of the plant through a process called polar auxin transport.
- Auxins control processes such as cell elongation, root and shoot growth, apical dominance, tropisms (response to directional stimuli), and root initiation.

2. Root initiation and the role of IBA:
- Root initiation refers to the formation of new roots from plant tissues.
- IBA is specifically involved in promoting root initiation and is commonly used as a rooting hormone in horticulture.
- When IBA is applied to plant cuttings or tissue cultures, it stimulates the formation of roots.
- IBA enhances the development of adventitious roots, which are roots that arise from non-root tissues, such as stems or leaves.
- It promotes cell division and elongation in the cells of the cambium or pericycle, leading to the formation of new roots.

3. Other hormones and their role in root initiation:
- While IBA is primarily responsible for root initiation, other plant hormones also play a role in this process.
- Gibberellins (GA3) promote stem elongation and cell division but are not primarily involved in root initiation.
- Abscisic acid (ABA) regulates seed dormancy, stomatal closure, and stress responses in plants but does not have a major role in root initiation.
- Kinetin is a type of cytokinin hormone that promotes cell division and growth, but its role in root initiation is secondary to that of auxins.

In conclusion, IBA is the hormone primarily concerned with root initiation. It promotes the formation of adventitious roots by stimulating cell division and elongation in plant tissues. Other hormones may have some influence on root initiation, but IBA is the key hormone in this process.