Correct answer is: C. The lowest-level vassal in a feudal system.
Under Feudalism, there was a peasant status called Serfdom. Serfs were bound and tied to the land, if sold they came with it. It was not slavery itself, but under the feudalism they were definitely the lowest form of "tenant" of the land.
The answer for your question is 1666 Kilometers
Both of these are important to emphasize in a textbook chapter, as both of them are fair representations of the role that communist policies had in the Soviet Union.
If we only reported on the achievements in space of the Soviet Union, then we would minimize the damage that certain policies caused. On the other hand, if we only reported on Stalin's Great Terror, then we would be painting a biased and inaccurate picture of the regime. Therefore, it is important to mention both of these events, as well as the many others that defined the Soviet Union during this time period.