Which of the following hormone types is LEAST likely to affect plant growth?

The hormone type that is least likely to affect plant growth is stimulants. Stimulants are generally associated with the enhancement of biological activity or metabolic processes primarily in animals and humans, rather than plants. They often refer to substances that can temporarily increase alertness or physical performance in animals, which is not a function related to growth regulation in plants.

In contrast, phytohormones, auxins, and gibberellins are all specifically involved in the regulation of plant growth. Phytohormones is an umbrella term that encompasses various hormones including auxins and gibberellins, both of which play critical roles in how plants grow and adapt. Auxins are primarily involved in cell elongation and directional growth, while gibberellins promote stem elongation, seed germination, and flowering. Thus, these three types of hormones are directly connected to plant growth, making stimulants the least relevant to this context.