The correct answer is Option D: NAA.

Here’s a brief explanation:

$IAA$ (indole-3-acetic acid) is the naturally occurring auxin in plants.

$GA$ (gibberellin) is a class of plant hormones involved in growth but is not an auxin.

$IBA$ (indole-3-butyric acid) is an auxin-like compound, but it is mostly considered a naturally derived rooting hormone.

$NAA$ (naphthaleneacetic acid) is a synthetic auxin widely used in plant propagation and agriculture.

So, among the options given, $NAA$ is the synthetic auxin.