Auxin is transported polarly. Mostly, auxin movement is

The correct option is basipetal.

Polar auxin transport (PAT) refers to the regulated, directional movement of the plant hormone auxin (indole-3-acetic acid) through plant tissues. This transport is active, energy-dependent, and occurs primarily in a specific direction rather than relying on simple diffusion through the vascular system.

In stems and coleoptiles, auxin is synthesized primarily in the young shoot apex, developing leaves, and apical buds. From these apical regions of synthesis, auxin moves downward through the parenchyma cells associated with vascular bundles toward the base of the plant. This unidirectional, downward movement from the apex (tip) to the base is biologically termed basipetal transport.

To understand the terms:
1. Basipetal movement: Transport directed toward the base (from morphologically young apical parts toward older basal parts). In shoots, this represents downward movement.
2. Acropetal movement: Transport directed toward the apex or tip (from base to apex). In roots, auxin transport near the central cylinder is acropetal (toward the root tip), but the predominant polar transport in the shoot system is basipetal.
3. Centripetal movement: Movement directed toward the center or inward from the periphery.

Consequently, because the primary and most prominent polar transport of auxin in the shoot system occurs from the shoot tip downward toward the base, auxin movement is characterized mostly as basipetal.