Which of the following definitions best describes auxin?

Auxin is defined as a naturally occurring plant hormone that plays a crucial role in promoting plant growth. This hormone is primarily responsible for cell elongation, apical dominance, and the process of tropisms, which allow plants to respond to environmental stimuli by growing towards light or away from gravity. Auxin influences various aspects of plant development, including root formation, flowering, and fruit development, making it essential for overall plant health and growth.

The other choices do not accurately define auxin. The option that mentions a plant chemical aimed at deterring disease organisms refers to a different group of substances known as phytoalexins, which help with plant defense rather than growth promotion. The mention of the capital of Texas and the USDA does not pertain to plant hormones at all, making it irrelevant to the question. Lastly, a chemical used as a plant growth retardant typically refers to substances that inhibit growth rather than promote it, thereby distinguishing it from the role of auxin in enhancing growth.