Final answer:
Plasmids are the structures used to transfer recombinant DNA.
Step-by-step explanation:
The correct answer is d. Plasmids. Plasmids are small, circular DNA molecules that are used to transfer recombinant DNA. They are commonly found in bacteria and can be manipulated and transferred between cells to introduce new genes or traits. Plasmids serve as vectors or carriers to introduce foreign DNA into bacterial cells for expression or replication.