The idea that people who live in Germany must be of Germanic heritage is an expression of A. Pan-nationalism
B. National factionalism
C. Ethnic nationalism
D. Governmental nationalism

The idea that people who live in Germany must be of Germanic heritage is an expression of ethnic nationalism.

Explanation:

Nationalism is defined by the feelings of people of a particular group that shares common characteristics, both physical and cultural, and they cherish, protect, annd highly value their heritage. Now nationalism as such is not something bad, it is actually good when it comes to preserving cultures, but a problem arises when nationalism becomes radicalized.

One of the more radical nationalisms on the spectrum is ethnic nationalism. This type of nationalism is based on the idea that everyone that lives or can live in a particular country is or has to be of the descent of the ethnic group that is dominant there, in this case, that being the German people. This leads to the discrimination of other groups of people, denying their heritage, and often being expressed with aggressive actions. In reality, this type of nationalism doesn't even make any sense as all modern-day ethnic groups (except some isolated tribes) are a mixture of several different ethnic groups from the past. In the case of Germany for example, it is not even the Germanic genes that dominate, but the Celtic ones, and there are also relatively high percentages of Slavic genes present among the modern-day population as well.