Explanation:
NAA (Naphthalene Acetic Acid) is a synthetic auxin, which is a plant growth regulator used to promote rooting and other growth processes.
Other options:
IBA (Indole-3-butyric acid): A naturally occurring auxin, though it can be synthesized in labs.
IAA (Indole-3-acetic acid): A naturally occurring auxin found in plants.
GA (Gibberellic acid): A different class of plant hormones called gibberellins, not auxins.
Thus, NAA is the correct answer.