International migration refers to the movement of people from one country to another for the purpose of establishing a new residence. It involves the crossing of international borders and can occur for various reasons, such as economic opportunities, education, family reunification, or seeking refuge from conflict or persecution.

Here is a detailed explanation of why option 'C' is the correct answer:

1. Definition of international migration:
- International migration is the process of individuals or households moving across international borders to establish a new residence in a different country.
- It involves leaving one's home country and settling in a foreign country.

2. Reasons for international migration:
- Economic opportunities: Many individuals migrate to other countries in search of better job prospects, higher wages, and improved living conditions.
- Education: Students often migrate to other countries to pursue higher education and gain access to quality academic institutions.
- Family reunification: People may migrate to join their family members who are already residing in another country.
- Refugees and asylum seekers: Those fleeing conflict, persecution, or natural disasters in their home countries often seek refuge in other countries.

3. Distinction from other types of migration:
- National migration (option 'A') refers to the movement of individuals within the same country. It does not involve crossing international borders.
- Internal migration (option 'B') refers to the movement of individuals within the boundaries of a particular country. It can involve movement between different regions or states within the same country.
- Foreign migration (option 'D') is not a commonly used term to describe migration patterns. It does not adequately capture the concept of individuals leaving or entering a country.

In conclusion, international migration (option 'C') is the appropriate term for the movement of people across international borders, as it encompasses the various reasons and circumstances under which individuals migrate from one country to another.