The correct option is C Shifting cultivation
Ex situ plant conservation means 'off-site conservation'. It is the 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 the population to a less threatened location.
Field gene banks are place where the 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, then abandoned and allowed to revert to their natural vegetation while the cultivator moves to another plot, which results in deforestation and not conservation
Thus, the correct answer is option (C), 'Shifting cultivation'.