Beginning in the nineteenth century, industrializing lands experienced a social change known as the demographic transition when?

When fertility rates fell.

Explanation:

The demographic transition consists in the change of the demographic profile of the population of a city or country. Demographic transitions are usually triggered by industrialization and urbanization.

Pre-industrial, agrarian societies have a demographic profile of very high fertility rates, very high child mortality, and very low life expectancy.

In other words, in agrarian societies, women have many children, only a few of those children survive into adulthood, and because life expenctacy is low, the average person is young, and there are very few old people.

As the agrarian society becomes more industrial and more urban, women start having fewer children, and more of those children survive, because the increased wealth generated by the industrial economy allows better medical care and nutrition.

Life expectancy rises as well, leading to an general aging of society. For example, in Japan, a highly industrialized, and very urban country, the average age is 46 years.