The inactive protoxin gets converted into an active form due to the :-

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

Option (1) Alkaline pH of insect’s gut — Correct. The Bt toxin is kept as an inactive protoxin in crystals. In the insect’s gut the alkaline pH dissolves the crystals and activates the toxin.

Option (2) Temperature and acidic pH of gut — Incorrect. The pH is reversed: the gut is alkaline, and temperature plays no part.

Option (3) Exposure to light — Incorrect. The activation happens inside the gut, not by light.

Option (4) Exposure to light and acidic pH of gut — Incorrect. Light plays no part in activating the toxin; the change takes place inside the insect’s gut. The gut is also alkaline, not acidic. So both parts of this option are wrong.

Note: A similar question was also asked in CUET (UG) 2025 on 28-05-2025 (Slot 1) and on 26-05-2025 (Slot 2), and in CUET (UG) 2022 on 30-08-2022 (Slot 2).

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.”