Final answer:
Transmigration is most common in Indonesia, where it is a state-sponsored policy aimed at redistributing the population to manage resources and reduce economic disparities, although it has led to ecological and social issues.
Step-by-step explanation:
The southeast Asian country where transmigration is most common as a policy of the national government is Indonesia. Transmigration is a policy that involves the movement of people from densely populated regions to less populous areas of the country. This policy aims to achieve a more balanced demographic distribution and has been implemented to alleviate overpopulation in certain regions while promoting development in others. An example of this would be the relocation of people from the densely populated island of Java to less populated areas, such as Borneo and Papua.
Historically, transmigration has helped in managing resource distribution and reducing regional economic disparities. However, it has also led to issues such as deforestation, land disputes, and inter-communal conflicts between migrants and indigenous populations.