The system that evolved in Europe out of the collapse of the Roman Empire was

Imperialism refers to the policy of extending a country's power and influence through colonization, military force, or other means. While the Roman Empire practiced imperialism, the post-Roman period saw a decline in territorial expansion and a focus on localized governance, making this choice irrelevant to the immediate aftermath of the empire's collapse.

Absolutism is a political doctrine where a single ruler holds absolute power, often associated with later monarchies in Europe. This system became more prominent in the 17th century, well after the feudal structure took root following the fall of the Roman Empire, and thus does not represent the initial response to that collapse.

Republicanism is a political ideology centered on the principles of a republic, where representatives are elected to govern. This system was more reflective of earlier Roman governance and did not gain traction after the fall of the empire, as Europe shifted towards feudal allegiances and local power structures.

Protocapitalism refers to early forms of capitalist practices that began to emerge in the late Middle Ages and Renaissance. This economic system is not representative of the immediate post-Roman period, which relied heavily on feudal relationships rather than market-driven economies.