Which one of the following is an example of carrying out blological...

Bt cotton is Genetically Modified (GM) cotton which has an incorporated gene extracted from the bacterium Bacillus thuringiensis. This gene codes for $B t$ toxin in plant tissues which is harmful only to a small fraction of insects, most notably the larvae of lepidopterans, moths, butterflies, beetles, flies, etc and harmless to other forms of life. So, it is used as biological control of pests/diseases.