Which part of the insect is damaged by active toxin of Bacillus thuri…

The correct answer is Option (3) → Digestive tract

Option (3) Digestive tract — Correct. The Bt toxin is an inactive protoxin in the bacterium. In the alkaline gut of the insect it becomes active, binds to the surface of the midgut epithelial cells and makes pores in them; the cells swell and burst, so the digestive tract is damaged and the insect dies.

NCERT Extract:
Biotechnological Applications in Agriculture – “… Bt Cotton: Some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes). B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein. … Actually, the Bt toxin protein exist as inactive protoxins but once an insect ingest the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut which solubilise the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect.”