I would go with B, "populations declined in urban areas"
Explanation:
For all 3, the urban population is much greater in 100 CE than 1000 CE. I find the graphs to be set up in a poor way and you'd expect them to have grown, so forgive me if I'm wrong.
As you can see in the graph over the course of those 9 centuries the population decline, in different measurements in the three metropolis that we are seeing in Rome, the population became almost 1/20 of that of the first cenury compared with the population of 10 centuries after that, in other areas, like london and paris the declining of the population was less drastic but still observed a decline on urban population.
