The statement that best describes how auxin molecules act to cause plant tropisms is: "They congregate on the side toward the stimulus."
Auxin is a plant hormone that regulates growth and development. When a plant is exposed to a directional stimulus, such as light or gravity, the distribution of auxin in the plant changes. In response to a positive stimulus, such as light, auxin accumulates on the shaded side of the plant, promoting growth on that side and causing the plant to bend towards the light. In response to a negative stimulus, such as gravity, auxin accumulates on the lower side of the plant, promoting growth on that side and causing the plant to bend upwards.