Which demographic indicator would best align with high educational attainment in a more developed country?

Demographic indicators of income status.

Explanation:

The education attainment of the nation such as developing or developed ca be seen in terms of higher standards of living and describes the schooling, along with the teachers and students. A higher-income status would best indicate the level of education as many countries don't have money to continue for higher studies. Thus are unable to take higher education and are school dropouts.

Why you put last one a? i think is b.