本题考查加拿大人口最多的地区。

Westward from the Atlantic, between the Appalachians and the Canadian Shield, lies the region known as the The Great Lakes-St. Lawrence Lowlands. ... The richness of its soils has helped make this region the most populous in Canada and one of the country's main agricultural regions.

从大西洋向西，在阿巴拉契亚山脉和加拿大地盾之间是著名的圣劳伦斯低地。 ...其丰富的土壤使这里成为加拿大人口最多的地区，它还是国家主要的农业地区。

故本题选D。