What is a common trait among developed nations?
a multicultural society
a national language
a high per-capita income
a highly agricultural society

The correct answer here is: a high per-capita income.

Explanation:

When talking about the level of development of nations, there are several characteristics that must be taken into account, in order to consider a nation developed, or in the process of development. As such, these characteristics are: low incidence of poverty, high standard of living, less income inequalities, lower levels of unemployment, lesser levels of deaths during birth and, most commonly, these developed countries have people with high per-capita incomes, when compared with less developed nations.

no questions on whether they are allowed that right