Final answer:
The best way to demonstrate thigmonastic behavior in a plant is by c) Touching the plant's leaves gently, such as those of a Mimosa pudica, which quickly responds by folding inwards.
Step-by-step explanation:
c) Touching the plant's leaves gently. Thigmonastic behavior refers to a plant's response to touch or mechanical stimulation. To demonstrate this phenomenon in a lab setting, the most suitable experiment is option c: touching the plant's leaves gently. Thigmonastic responses are often observed in certain plants that exhibit rapid movements or changes in leaf orientation upon physical contact. Mimosa pudica, commonly known as the sensitive plant, is a classic example. When touched, its leaflets fold inwards as a protective response. By gently touching the leaves, the lab teacher can illustrate the thigmonastic behavior, showcasing how plants can respond to mechanical stimuli in their environment. Options a, b, and d are less relevant to thigmonastic behavior and may involve other plant responses.