To solve this problem, we need to understand why a green plant bends towards the light when exposed to light on only one side. This phenomenon is known as phototropism. Let's analyze the options:$\newline \bullet$Option 1: Green plants need light to perform photosynthesis.$\newline$ - While it is true that plants need light for photosynthesis, this does not explain the bending towards the light.$\newline$ - Photosynthesis occurs in the chloroplasts and is not directly responsible for the bending movement.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 2: Green plants seek light because they are phototropic.$\newline$ - Phototropism is the phenomenon being described, but this option does not explain the mechanism behind the bending.$\newline$ - It simply restates the observation without providing a causal explanation.$\newline$ - Therefore, this option is not the best choice.$\newline \bullet$Option 3: Light stimulates plant cells on the lighted side to grow faster.$\newline$ - This is incorrect because the cells on the lighted side do not grow faster.$\newline$ - In fact, the cells on the shaded side elongate more, causing the plant to bend towards the light.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 4: Auxin accumulates on the shaded side, stimulating greater cell elongation there.$\newline$ - This is the correct explanation.$\newline$ - Auxin is a plant hormone that promotes cell elongation.$\newline$ - When a plant is exposed to light on one side, auxin redistributes to the shaded side.$\newline$ - The higher concentration of auxin on the shaded side causes those cells to elongate more than the cells on the lighted side.$\newline$ - This differential growth causes the plant to bend towards the light.$\newline$Therefore, the correct option is Option 4.