Assertion (A) : Insect resistant transgenic cotton has been produced by inserting Bt gene. Reason (R) : The Bt gene is derived from a bacterium.

Assertion is true: insect‑resistant transgenic cotton has been produced by inserting Bt genes such as Cry1Ac into cotton, a technology commercialized in India (Bt cotton hybrids with Cry1Ac were approved and cultivated from 2002) [1]. Reason is true and correctly explains the assertion: the Bt genes originate from the bacterium Bacillus thuringiensis and encode crystal (Cry) insecticidal proteins; when these genes are incorporated into the plant genome the plant expresses Cry proteins that are toxic to target caterpillar pests (e.g., bollworms), conferring insect resistance [2]. Thus R is the correct mechanistic explanation for A: the bacterial Bt gene imparts insecticidal activity when expressed in cotton.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Bt. Cotton in India > p. 40
2. [2] https://www.ars.usda.gov/ARSUserFiles/oc/np/btcotton/btcotton.pdf