The $B t$ toxin in genetically engineered $B t$ cotton kills the pest by:

Option (1) is the correct answer because the activated toxin binds to the surface of midgut epithelial cells and creates pore that cause cell swelling and lysis and eventually cause death of the insect. $B t$ toxin doesn't kill pest by affecting their respiratory system, nervous system and it doesn't alter the pH of body fluids. So, options (2), (3) and (4) are not the answer.