The correct option is D Nosema
Nosema is the genus of small, unicellular parasites recently re-classified as a fungus, that mainly affects honey bees.
The disease is called as pebrine. It is caused by microsporidian, Nosema bombycis. It is one of the most dangerous and devastating disease in silkworm. This disease devastated sericulture industry in several countries and even it wiped out the industry from some of the European countries.