Which one of the following is a natural auxin?

IAA (Indole-3-acetic acid) is the principal, most widespread naturally occurring auxin synthesized by plants to regulate growth, cell elongation, and apical dominance. On the other hand, NAA (Naphthaleneacetic acid) and $2,4\text{-D}$ ($2,4\text{-Dichlorophenoxyacetic acid}$) are purely synthetic alternatives.