Transport of auxin is

The correct option is "polar".

Auxin (specifically indole-3-acetic acid, or IAA) is a vital plant hormone that regulates various aspects of plant growth and development, including cell elongation, apical dominance, and tropisms.

Unlike most other plant hormones which move passively or non-directionally through the vascular system, the transport of auxin is highly regulated, unidirectional, and active. This unidirectional movement is referred to as polar transport.

In stems and coleoptiles, auxin is transported in a basipetal direction—meaning it moves from the shoot apex (where it is primarily synthesized) downward toward the base. In roots, the transport is more complex, involving both acropetal movement (toward the root tip) and basipetal movement (away from the root tip). This coordinated, directional movement is mediated by specialized auxin influx carriers (such as AUX1/LAX) and efflux carriers (specifically PIN proteins) that are localized asymmetrically on the plasma membranes of plant cells, ensuring that auxin flows in a specific, polar direction.