In Bt cotton, the Bt toxin present in plant tissue as pro-toxin is converted into active toxin due to:

Step-by-step solution

Bacillus thuringiensis which contain cry protein in inactive form known as pro-toxin. When this is ingested by insect it get activated by alkaline ph of gut, which solubilize the protein crystal. This activated toxin bind to the epithelial midgut cell result into lysis of epithelial cell and eventually lead to the death of insect.