Final answer:
The linking number remains the same when a plasmid DNA with 20% changed to Z-DNA is treated with a type IA topoisomerase, as this enzyme cleaves only one strand of DNA to relieve or introduce twists, but does not change the overall linking number. The correct option is C.
Step-by-step explanation:
When 20% of the DNA in a relaxed circular-covalently-closed plasmid changes from B-DNA to Z-DNA and is treated with an enzyme that has type IA topoisomerase activity, the linking number of the plasmid will ultimately remain the same.
Type I topoisomerases, including type IA, function by cleaving one strand of the DNA, allowing unwinding or rewinding of the DNA to relieve or introduce twists, and then re-ligating the cleaved strand.
Because the linking number only changes when both strands are simultaneously cleaved (as done by type II topoisomerase), and since IA topoisomerase does not perform this action, the overall topological property of the linking number is preserved, although transiently it may be altered during the enzyme's action.