Bacillus thuringiensis produces proteins (Bt toxin) that kill certain…

The correct answer is Option (3) → (A), (B), and (D) only

(A) Bt toxin protein exists as an inactive form (protoxin) in Bacillus thuringiensis. — Correct. The toxin is kept inactive inside the bacterium, which is why it does not kill the bacterium itself.

(B) Bt toxin protein exists as an active form toxin in the insect gut. — Correct. Once eaten, the protoxin becomes active in the gut. It binds to the midgut cells and makes pores, and the insect dies.

(C) The alkaline pH of the gut of an insect solubilize the toxin and converts in an inactive form. — Incorrect. The alkaline pH does dissolve the crystals, but it converts the toxin into the active form, not the inactive form.

(D) The toxin is coded by a gene cryIAc named cry. — Correct. The Bt toxin genes are the cry genes; cryIAc and cryIIAb control the cotton bollworm, and cryIAb the corn borer.

NCERT Extract:
Biotechnological Applications in Agriculture – “… 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. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect. Specific Bt toxin genes were isolated from Bacillus thuringiensis and incorporated into the several crop plants such as cotton (Figure 10.1). The choice of genes depends upon the crop and the targeted pest, as most Bt toxins are insect-group specific. The toxin is coded by a gene cryIAc named cry.”