Toxin of Bacillus thuringiensis is activated in insect by :

The correct answer is Option (1) → Alkaline pH of gut

Option (1) Alkaline pH of gut — Correct. The toxin is made as an inactive protoxin crystal. In the insect gut the alkaline pH dissolves the crystals and activates the toxin, which binds to the gut cells, makes pores and kills the insect.

Option (2) Acidic pH of gut — Incorrect. This reverses the condition: the activation needs an alkaline, not acidic, gut.

Option (3) High temperature — Incorrect. NCERT says the inactive protoxin becomes the active toxin because of the alkaline pH of the insect gut, which dissolves the crystals. Temperature is not the cause.

Option (4) Mechanical action of gut — Incorrect. The crystals are dissolved chemically by the pH, not broken by movement of the gut.

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.”