The correct answer is (2) cry 1Ac + cry 2Ab.

These two Bt genes from Bacillus thuringiensis are stacked in Bollgard II cotton to provide broad and durable resistance against major lepidopteran pests including cotton bollworm, tobacco bollworm and pink bollworm.

Option-by-option explanation

1. cry 1Ab + cry 1Ac – Incorrect

• Both are cry1 family toxins, and this pair is not the commercial combination used in Bollgard II cotton.

• Stacking only cry1-type genes would give narrower toxin diversity than cry1 + cry2 stacking.

2. cry 1Ac + cry 2Ab – Correct

• Bollgard II cotton was developed by adding the cry2Ab gene to existing Bollgard cotton that already expressed cry1Ac.

• The two toxins have overlapping but distinct spectra and binding sites, improving control of bollworms and delaying resistance evolution.

3. cry 1Ab + cry 2Ab – Incorrect

• This specific pair is not the standard Bollgard II construct; the marketed Bollgard II event uses cry1Ac, not cry1Ab, together with cry2Ab.

4. cry9C cry 1Ab – Incorrect

• cry9C has been associated with some Bt maize events, not Bollgard II cotton.

• This combination is not used in the commercial Bollgard II product targeting cotton bollworm complexes.

SEO-oriented introduction (for article use)

Bollgard II cotton is a second-generation Bt cotton that combines the insecticidal genes cry1Ac and cry2Ab to provide strong and durable resistance against key lepidopteran pests such as cotton bollworm, tobacco bollworm, and pink bollworm. By stacking these two Bt toxins with different modes of action, Bollgard II cotton broadens the pest-control spectrum and slows resistance development compared with single-gene Bt lines.