Concept:Nematodes are microscopic roundworms that mainly attack plant roots, causing root diseases.Explanation:Root knot disease of tomato is caused by root-knot nematodes of the genus Meloidogyne.These nematodes infect the roots and produce swellings or galls called knots.Fusarium wilt and leaf spot are fungal diseases.Leaf curl of tomato is usually caused by viruses or insect pests, not nematodes.Therefore, the nematode-caused disease among the options is root knot.Answer:B. root knot