To solve this problem, we need to understand why auxin is used by gardeners to prepare weed-free lawns without damaging the grass. Let's analyze the options:$\newline \bullet$Option 1: Auxin does not affect mature monocotyledonous plants.$\newline$ - Auxins are a class of plant hormones that primarily affect dicotyledonous plants.$\newline$ - Grasses, which are monocotyledons, are less sensitive to auxins compared to dicots.$\newline$ - Therefore, when auxins are used as herbicides, they selectively target broadleaf weeds (dicots) without harming the grass (monocots).$\newline$ - This makes Option 1 correct.$\newline \bullet$Option 2: Auxin can help in cell division in grasses, to produce growth.$\newline$ - While auxins can promote growth, this option does not explain why auxins do not damage grass when used as herbicides.$\newline \bullet$Option 3: Auxin promotes apical dominance.$\newline$ - Apical dominance is the suppression of lateral bud growth by the apical bud.$\newline$ - This is unrelated to the selective action of auxins on weeds versus grass.$\newline \bullet$Option 4: Auxin promotes abscission of mature leaves only.$\newline$ - Auxins generally prevent abscission rather than promote it.$\newline$ - This option does not explain the selective action of auxins on weeds.$\newline$Therefore, the correct explanation is that auxin does not affect mature monocotyledonous plants, which corresponds to Option 1.