Final answer:
In rDNA technology, a plasmid vector is cleaved by the same restriction enzyme that cleaves the donor gene to ensure compatible sticky ends for successful ligation.
Step-by-step explanation:
In rDNA technology, to prepare for the recombination process, a plasmid vector is digested with a restriction enzyme to create an opening in the DNA circle. The foreign DNA to be inserted is also cut by the same restriction enzyme to ensure that they have compatible sticky ends for successful ligation. Hence, the answer to the student's question is that a plasmid vector must be cleaved by (b) the same enzyme that cleaves the donor gene. This ensures that the complementary sticky ends of the plasmid and the foreign DNA can anneal or join together correctly. Following this, DNA ligase is used to seal the DNA fragments, creating a recombinant DNA molecule.