Final answer:
The lowest period for a country's economy and GDP is called a depression, which is an extended and severe form of the recession that significantly decreases income and employment.
Step-by-step explanation:
The lowest period for a country's economy and GDP is called a depression. Recession refers to a period where real GDP decreases for at least six months consecutively, and typically the unemployment rate increases during this time. A depression is an extension of this, characterized by being particularly lengthy and severe, leading to a significant decline in income and employment. An example of a depression is the 1930s Great Depression. On the other hand, an upswing in economic activity is referred to as an expansion or boom. Thus, the correct answer to fill in the blank is B. Depression.