Final answer:
The beginning of demographic transition is characterized by declining death rates while birth rates stay high, leading to increased population growth.
Step-by-step explanation:
The characteristic of the beginning of demographic transition is B. declining death rates, which leads to a period of increased population growth due to high birth rates.
In the first stage of demographic transition, both birth and death rates are high, resulting in slow population growth. As the transition begins, improved medical care and living standards typically cause a decline in the death rate. Consequently, the population starts to grow more rapidly because the birth rate initially remains high. Over time, as the society continues to develop and transform, the birth rate begins to fall, leading to slower population growth and eventually reaching a point where the birth rate and death rate stabilize, as seen in later stages of the demographic model.