What triggers activation of protoxin to active Bt toxin of Bacillus thuringiensis in boll worm?

Correct option is C. Alkaline pH of gut. Bacillus thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein. These proteins exist as inactive protoxins but once an insect ingests the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut which solubilizes the crystals. The activated toxin binds to the surface of midgut epithelial cells and creates pores that cause cell swelling and lysis and eventually cause the death of the insect. So, the correct answer is "Alkaline pH of gut".