Which form of internal migration has affected most countries?

The correct answer is: rural to urban migration.

Rural to urban migration is the large-scale movement of people from rural areas to urban areas in search of better opportunities and a higher standard of living. This form of internal migration has been a significant trend in many countries around the world, particularly in developing countries. It has had profound impacts on both rural and urban areas and has shaped the demographic, social, economic, and political landscapes of many regions.