Answer
That plastid DNA should be overall closer to that of cyanobacteria than to nuclear DNA of cells which have plastids, though transfer of DNA between nuclei and plastids might mean that some regions of plastid DNA would be closer to nuclear DNA.
Work Step by Step
To see this, remember that overall DNA similarity relates to the closeness of phylogenetic relationships. However (review earlier material on horizontal gene transfer), some plastid DNA or nuclear DNA might have been horizontally transferred back and forth, confusing the issue for some small regions.
