Which one of these is commonly used synthetic auxin

The correct answer is NAA.

Auxins are a class of plant hormones (or plant growth regulators) that play an essential role in coordinating many growth and behavioral processes in plant life cycles, such as stem elongation, root initiation, and bud growth. Auxins can be broadly classified into two categories: natural auxins and synthetic auxins.
1. Natural Auxins: These are naturally synthesized by plants. The most common and well-known natural auxin is Indole-3-acetic acid (IAA). Another naturally occurring auxin is Indole-3-butyric acid (IBA), which is also found in plants (though it can be synthesized artificially for commercial use as well).
2. Synthetic Auxins: These are chemically synthesized compounds that mimic the physiological activities of natural auxins. Naphthalene acetic acid (NAA) is a classic example of a widely used synthetic auxin. It is commonly employed in agricultural and horticultural practices to facilitate rooting in stem cuttings, prevent premature fruit drop, and induce flowering.

Let us look at the options provided in the question:
- NAA (Naphthalene acetic acid): A synthetic auxin.
- IBA (Indole-3-butyric acid): A naturally occurring auxin isolated from plants (also used synthetically).
- IAA (Indole-3-acetic acid): The primary natural auxin found in plants.
- GA (Gibberellic Acid): This belongs to a completely different class of plant hormones called Gibberellins, which are involved in stem elongation, germination, and flowering, and is not an auxin.

Therefore, NAA is the synthetic auxin among the choices provided.