Which type of herbicide affects plant cell elongation during the growing season?

The herbicide that affects plant cell elongation during the growing season is an anti-gibberellin or gibberellin inhibitor. Gibberellins are a class of hormones in plants that play a critical role in promoting growth and development, particularly through cell elongation. By inhibiting gibberellin activity, these herbicides disrupt the normal growth processes, leading to stunted plant development. This action is particularly effective against certain types of unwanted plants that rely on gibberellin for their growth.

The other types of herbicides have different modes of action. Auxin-type herbicides mimic natural plant hormones that can also cause abnormal growth, but they function differently than gibberellin inhibitors. Selective herbicides target specific weeds without harming the desired plants, but they do not specifically focus on cell elongation. Contact herbicides work by affecting the parts of the plant that they directly touch, leading to immediate damage rather than influencing internal hormonal processes. Thus, among the provided options, anti-gibberellins or gibberellin inhibitors specifically impact the mechanism of cell elongation, making them the correct choice.