The plant hormone used to destroy weeds in a field is :

The question asks which plant hormone is used to destroy weeds in a field.

Plant hormones that affect growth are called auxins. When a synthetic auxin is applied in high concentrations, it causes uncontrolled growth, leading to the death of broad‑leaf weeds while leaving grasses relatively unharmed.

Among the listed options, 2, 4‑D (2,4‑dichlorophenoxyacetic acid) is a well‑known synthetic auxin herbicide. It mimics the natural auxin indole‑3‑acetic acid (IAA) but is far more potent, causing abnormal cell elongation and eventually killing the weed.

The other choices—IBA (indole‑3‑butyric acid), IAA (indole‑3‑acetic acid), and NAA (naphthalene‑acetic acid)—are natural or synthetic auxins used primarily to promote rooting or regulate growth, not as herbicides.

Therefore, the correct answer is 2, 4‑D.