Concept:African marigold acts as a natural soil pest suppressant, especially against nematodes.
Explanation:Marigold roots release chemical compounds into the soil.
These compounds are toxic to or repel nematodes.
Nematodes are microscopic roundworms that attack crop roots.
When marigold is planted between crops, the root exudates create a protective zone.
This reduces nematode populations in the soil.
It reduces the need for chemical nematicides.
It also protects the main crop from root damage and disease.
The other options, bacteria, viruses, and fungi, are not directly suppressed by marigold root chemicals.
Therefore, planting African marigold is specifically a method of nematode control.
Answer:D. nematodes