The correct option is C thuringiensis
An example of microbial biocontrol agents that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often written as Bt). These are available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees, which are attacked by the insect larvae. In the gut of the larvae, the toxin is released and the larvae get killed.