Final answer:
The demographic transition is a historical process where family sizes decrease due to falling birth rates in industrialized nations, a trend first observed in the late 1800s.
Step-by-step explanation:
The term demographic transition refers to the phenomenon of shrinking family sizes due to decreased birth rates in industrialized nations, primarily observed in the late 1800s. This concept is closely linked to the social and economic changes that occurred during the industrial revolution, where there was a shift from agricultural livelihoods to urban industrialized work. The demographic transition has several stages, starting with high birth and death rates, progressing to high birth rates and lower death rates (due to improved healthcare), and eventually leading to both low birth and death rates as families tend to have fewer children.
To address the choices provided, demographic transition refers to neither the decrease in skilled crafts (a), the decrease in the age of first marriage (b), the reform movement to end slavery in the United States (c), nor the decline of cotton production (d). Instead, it describes the broad changes in population dynamics as a result of industrialization and modernization.