Weedicides are chemical substances used to control or eliminate unwanted weeds. Among the options given, 2,4-D is a commonly used herbicide that effectively targets weeds. Let's take a closer look at each of the options and explain why 2,4-D is a weedicide.

1. IAA (Indole-3-acetic acid):
IAA is a natural plant hormone called auxin, which plays a crucial role in plant growth and development. While it is involved in various physiological processes, IAA is not primarily used as a weedicide. Instead, it is commonly used in plant tissue culture and plant growth regulators.

2. 2,4-D (2,4-Dichlorophenoxyacetic acid):
2,4-D is a synthetic herbicide that belongs to the phenoxy group of herbicides. It is widely used as a selective weedicide to control broadleaf weeds in various crops, lawns, and non-crop areas. 2,4-D mimics the action of the plant growth hormone auxin, causing uncontrolled growth in susceptible plants. It specifically targets broadleaf weeds while sparing grasses, making it an effective weedicide for many situations.

3. IBA (Indole-3-butyric acid):
Similar to IAA, IBA is also a natural auxin hormone found in plants. It is commonly used as a rooting hormone in horticulture to promote root formation in cuttings. While it has some herbicidal properties, IBA is not primarily used as a weedicide.

4. NAA (Naphthaleneacetic acid):
NAA is another synthetic auxin hormone that is primarily used as a plant growth regulator. It is often used to promote fruit set, prevent premature fruit drop, and induce rooting in cuttings. Although NAA has some herbicidal properties and can be used to control certain weeds, it is not widely used as a standalone weedicide.

In summary, among the options provided, 2,4-D is the weedicide. It is a synthetic herbicide that effectively targets broadleaf weeds while sparing grasses. The other options, IAA, IBA, and NAA, are all natural or synthetic auxin hormones primarily used as plant growth regulators and rooting hormones.