To solve this problem, we need to identify what triggers the activation of protoxin to active Bt toxin in Bacillus thuringiensis when it affects the boll worm.$\newline$Let's analyze the options:$\newline \bullet$Option 1: Body temperature - This is not the correct trigger for the activation of the Bt toxin.$\newline \bullet$Option 2: Moist surface of midgut - While the midgut is involved, moisture alone does not activate the toxin.$\newline \bullet$Option 3: Alkaline pH of gut - The alkaline pH in the gut of the boll worm is the correct trigger for the activation of the protoxin to the active Bt toxin.$\newline \bullet$Option 4: Acidic pH of stomach - This is incorrect as the activation occurs in an alkaline environment, not acidic.$\newline$The correct answer is Option 3: Alkaline pH of gut.$\newline$This corresponds to the correct option, which is Option 3.