To kill fungus on crops, we generally use —

Concept

Crop-protection chemicals are named after the pest group they act on: herbicides act on weeds, rodenticides on rodents, insecticides on insects, and fungicides on fungi. "Pesticide" is the umbrella term for this entire family of pest-control agents, not a single agent within it.

Application

The stem asks specifically for the agent generally used to kill fungus on crops. Among the four options, only the fungus-targeted agent is formulated to disrupt fungal spore germination and cell function — insecticides act on insect physiology and pesticides simply names the whole class, so neither is the specific fungus-killing agent.

Cross-check

• Insecticides — disrupt the insect nervous system or exoskeleton; they have no action on fungal cells, so they cannot control crop fungal disease.

• Pesticides — the broad umbrella category covering insect-, weed-, rodent-, and fungus-control agents alike; too general to be the one specific substance named for fungus control.

• All of these — fails because the insect-targeted agent and the umbrella-category term in this set do not themselves act on fungi, so the set is not uniformly correct.

Fungicides are the agents specifically formulated to kill or inhibit fungal growth, which is why they are the standard substance used to control fungus on crops.