Because regions are made by humans, A) everyone agrees on where regions start and end.
B) maps can never show regions at all.
C) you might not be able to tell when you’re leaving one region.
D) all regions from the past look exactly like regions of today.

A) is not the answer because people have always been fighting over who owns what land, or whether a piece of land is part of a nation or not.

B) is not the answer because, if it were we wouldn't have very accurate maps. Also, I don't

know about you, but it's generally hard for me to give verbal directions without a map to give a visual representation.

C) is the answer because, while there are signs, telling you that you're entering another region, sometimes your brain just ignores it, or maybe you didn't see it or something. Also going back to answer choice A), it's not always clear which area of land falls into whatever region.

D) is not the answer because come on, Pangea happened. Literally because of tectonic plates moving, all regions known today have moved, and are still moving as we speak.