What is a recession? a very mild and short expansion

a period when the economy is neither expanding nor contracting

a long and severe depression

a period when real GDP falls for at least six months

a period when real GDP falls for at least six months

Explanation:

A recession is a given period in which a particular country or continent is experiencing an economic downturn often within a period of 6months.

It is often caused as a result of a reduction in the level of commerce and industrial development which in turn lead to falling in the GDP of that country.

Hence, in this case, the correct answer is that the Recession is "a period when real GDP falls for at least six months."