Answer: C bacterial plasmid
A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes.
It can replicate in the cell separately . The desired gene(which codes for the desired trait) is inserted with this DNA molecule(plasmid) and tranfers to the host, where it gets replicate and helps in spreading the trait.
The insect resistent is caused in cotton in the same way. The gene for the resistant for insects is introduced into the bacterial plasmid which then transfers to the cotton. where it gets replicated and spread the resistance for insects.