Which of the following don't occur naturally?

Step-by-step solution

Auxins are one the growth promoters found in plants. These are mostly produces by the apical parts of roots and stems and from there move to different parts of plant as per need. Indole-3-acetic acid(IAA) and indole butyric acid(IBA) are produced by plant and have been isolated as natural auxins. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins that have been used in agricultural and horticultural practices.