Dicotyledonous weeds in agricultural fields are commonly controlled by using which selective herbicidal chemical among the following?

Introduction:
Weeds compete with crops for nutrients, water and light, reducing yield. Chemical herbicides are an important part of integrated weed management. Some chemicals selectively kill broad leaved dicot weeds while sparing narrow leaved monocot crops such as cereals. This question asks you to identify which chemical among the listed options is widely used as a selective herbicide against dicotyledonous weeds.

Given Data / Assumptions:
- The target plants are dicotyledonous weeds in crop fields. - Options include indole-3-acetic acid (IAA), 2,4-dichlorophenoxyacetic acid (2,4-D), abscisic acid (ABA) and naphthalene acetic acid (NAA). - We assume common agricultural crops like wheat and rice, which are monocots, are intended to be spared. - The chemical must act as a selective herbicide rather than a normal growth promoting hormone.

Concept / Approach:
2,4-dichlorophenoxyacetic acid, commonly abbreviated as 2,4-D, is a synthetic auxin used widely as a selective herbicide. It mimics natural auxin action but in dicot weeds it causes uncontrolled growth, leaf curling and eventual death. Monocot grasses are less sensitive at recommended doses, so crops like wheat survive while dicot weeds are destroyed. IAA and NAA are auxin like growth regulators used mainly for promoting root initiation and fruit setting, not as herbicides. Abscisic acid is a stress hormone and does not function as a selective herbicide in this context. Hence, 2,4-D is the correct answer.

Step-by-Step Solution:
Step 1: Identify the need for a selective herbicide that kills dicot weeds but not monocot crops. Step 2: Recall that 2,4-D has been used for many years to control broad leaved weeds in cereal fields. Step 3: Recognise that IAA and NAA are generally used as growth promoters, for example in rooting of cuttings and prevention of fruit drop. Step 4: Note that abscisic acid regulates stomatal closure and seed dormancy under stress but is not a widely used herbicide. Step 5: Conclude that 2,4-dichlorophenoxyacetic acid (2,4-D) is the herbicide that specifically matches the description.

Verification / Alternative check:
Agricultural manuals and weed management chapters clearly mention that 2,4-D is a selective systemic herbicide effective against many broad leaved weeds in cereals, sugarcane and turf. It is among the most commonly cited examples of synthetic auxin herbicides in competitive exam syllabi. Safety guidelines, recommended doses and application methods are described specifically for 2,4-D, confirming its central role in dicot weed control.

Why Other Options Are Wrong:
Indole-3-acetic acid (IAA): Natural auxin that regulates plant growth and cell elongation; it is not used as a field herbicide for killing weeds. Abscisic acid (ABA): Known for its role in stress responses, stomatal closure and seed dormancy; it does not serve as a selective herbicide in standard agricultural practice. Naphthalene acetic acid (NAA): Synthetic auxin used in horticulture for rooting and fruit retention, not primarily as a dicot weed killer.

Common Pitfalls:
Since all the chemicals except ABA are auxin related, students may struggle to differentiate their practical uses. Some may mistakenly select IAA or NAA because they recognise them as plant growth regulators. The key is to remember that 2,4-D is specifically formulated and used as a herbicide, and that it appears in many exam questions paired with the phrase dicot weeds. Creating a mental link between 2,4-D and wide leaf weed control can greatly reduce confusion.

Final Answer:
Dicotyledonous weeds are commonly destroyed by the selective herbicide 2,4-dichlorophenoxyacetic acid (2,4-D).