Dear Student,
The statement, "The choice of genes depends upon the crop and the targeted pest, as most bt toxins are insect-group specific" tries to explain that there can be numerous gene coding proteins for different pests and for different crops. Like in this case of 'Bt toxins', there is a gene named cry, which code for this toxin. And now, there are number of proteins encoded by this like cryIAb which controls cotton bollworms and crIIAb which controls corn borer.
Regards.