The process by which a country's population becomes smaller because its birth rate and net migration are too low to replace those who die and emigrate is called

Population Shrinkage (or Population Decline)

Explanation:

It can be used to refer to long-term demographic trends, such as urban decay or rural exodus, but is also commonly used to describe large population reductions due to violence, disease or other disasters. A good example is Japan, where the young population is small in contrast to a huge elderly population. In this case, the Population Shrinkage is caused by the less of birth, and the populational control executed by the Japanese government.

answer; (d) one tectonic plate plunges beneath another; (most common,convergent boundaries)