Odd one out: Abscisic acid (Option B)

Explanation:
Plant hormones, also known as phytohormones, play a crucial role in the growth and development of plants. They regulate various physiological processes such as cell division, elongation, flowering, fruit ripening, and response to environmental stimuli. There are several types of plant hormones, including auxin, abscisic acid, cytokinin, and gibberellins.

1. Auxin:
- Auxin is a plant hormone that regulates various aspects of plant growth and development.
- It is involved in cell elongation, apical dominance, phototropism, gravitropism, and root initiation.
- It promotes the growth of stems, inhibits lateral bud growth, and regulates the growth of roots.
- It is synthesized in the apical meristem and transported downwards through the plant.

2. Abscisic Acid:
- Abscisic acid is a plant hormone that regulates various physiological processes, especially in response to stress conditions.
- It plays a crucial role in seed dormancy, drought tolerance, and stomatal closure.
- It inhibits cell growth, promotes leaf senescence, and regulates the closure of stomata to prevent water loss.
- Abscisic acid is synthesized in various plant tissues, including roots, leaves, and seeds.

3. Cytokinin:
- Cytokinin is a plant hormone that promotes cell division and regulates plant growth and development.
- It is involved in the regulation of shoot and root growth, leaf senescence, and delay of aging.
- Cytokinin promotes the development of lateral buds, stimulates shoot formation in tissue culture, and regulates nutrient distribution within the plant.
- It is synthesized in actively growing tissues, such as root tips and developing fruits.

4. Gibberellins:
- Gibberellins are a group of plant hormones that regulate various aspects of plant growth and development.
- They promote stem elongation, seed germination, flowering, and fruit development.
- Gibberellins are involved in the regulation of internode elongation, breaking seed dormancy, and inducing flowering.
- They are synthesized in young leaves, shoots, and developing seeds.

Conclusion:
Out of the given options, abscisic acid (Option B) is the odd one out because it functions primarily as a stress hormone, regulating responses to drought, while the other three hormones (auxin, cytokinin, and gibberellins) are involved in promoting growth and development under normal conditions.