Ex situ plant conservation means 'off-site conservation'. It is process of protecting an endangered species of plant outside its natural habitat.
The best method of maximizing a species chance of survival is by relocating part of population to a less threatened location.
Field gene banks are place where plant genetic resources are kept as live plants that undergo continuous growth and require maintenance and is a good option for conservation.
Seed banks stores seeds as a source for planting in case seed reserves elsewhere are destroyed or getting endangered.
Shifting cultivation is an agricultural system in which plots of land are cultivated temporarily, n abandoned and allowed to revert to ir natural vegetation while cultivator moves to anor plot, which results in deforestation and not conservation
Thus, correct answer is option (C), 'Shifting cultivation'.