Concept:The naturally occurring auxin in plants is indole-3-acetic acid, commonly abbreviated as
IAA.
Explanation:IAA is the most abundant natural auxin found in plants.
It is mainly synthesized in the apical meristems of stems and roots, young leaves, and developing seeds.
The amino acid tryptophan serves as the precursor for
IAA synthesis.
Options B, C, and D are not natural auxins: dichlorophenoxyacetic acid is a synthetic auxin, while florigen is related to flowering.
Hence, the correct option is A.
Answer:Indole acetic acid (
IAA) — Option A.