The correct answer is C.
The graph shows the evolution of the population between year 650 and 1450 in Italy, the British Isles and in a third group constituted by France together with the Low Countries (green function). This latter group, whose population reached its peak in 1340, reaching almost 20 million inhabitants, suffered a decreased in its figures which were lowered until approximately 12 million inhabitants in 1450. The most prominent reason for this decrease was the spread of the Black Death.