Final answer:
Demographic trends include patterns such as changes in life expectancy and birth rates in a population. Decreases in life expectancy or increases in birth rates leading to population growth are examples of such trends that are integral to the study of demography, specifically within the context of the demographic transition.
Step-by-step explanation:
A demographic trend can be defined as a pattern or change in the characteristics of a population over time. Among the options provided, B) A decrease in the average life expectancy in a population and C) A rise in the birth rate leading to population growth are considered demographic trends. These trends are related to vital demographic characteristics such as birth rates, death rates, and life expectancy, which are core components of the study of demography. For example, a decrease in the average life expectancy in a population could occur due to various factors such as increased prevalence of diseases or poor healthcare infrastructure, which would be reflected in a population's demographics.
Conversely, a rise in the birth rate which leads to population growth is another key demographic change. It can result from factors such as increased fertility rates or declines in child mortality rates due to improvements in nutrition and healthcare. This is part of the demographic transition, where development and economic security can lead to lower death rates, and with more children surviving, families tend to have fewer children overall.