The correct answer is option 1, Weedicide.

Weedicide is a substance used to control and eliminate weeds, which are unwanted plants that compete with crops for resources such as sunlight, water, and nutrients. 2, 4-D (2,4-dichlorophenoxyacetic acid) is a commonly used herbicide that belongs to the class of synthetic auxins. It acts by disrupting the growth and development of broadleaf weeds, making it an effective weedicide.

Options 2, 3, and 4 are incorrect. Vitamin is a nutrient required by living organisms, and 2, 4-D does not fulfill this function. Fertilizer is a substance applied to soil or plants to provide essential nutrients for growth but 2, 4-D is not a fertilizer. Insecticide is a substance used to kill or repel insects, but 2, 4-D is not designed for this purpose.

Therefore, the correct option is 1, Weedicide.