The answer is study of cork bark.
According to the process of elimination:
- The endosymbiotic theory explains that mitochondria and chloroplasts descended from the same type of bacteria - thus, mtDNA is important in investigation of endosymbiotic theory
- DNA present in mitochondria is important also for inheritance research and forensic science.
This leaves study of cork bark as the right choice, which is confirmed by knowing that mtDNA contains genes for some metabolic processes.