What is active transport used for in plants?

Short answer: to move sugar to the phloem

Long answer: Active transport is the movement of a substance against its concentration gradient (from low to high concentration). In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids.

Plants exist in a perpetual state of lower internal concentration of water due to transpiration, so the first and second choice are out. Sugars are manufactured in the leaves which are fed by the xylem, so the fourth choice is out. The remaining third choice is the answer because sugars manufactured by photosynthesis in the leaf cells are introduced into the phloem via active transport.