Concept:A developing country is identified by its low average income per person.
Explanation:The most reliable indicator is income per head, which means the average income earned by each person in the country.
If income per head is low, the general standard of living is also low.
Developing countries usually have low incomes, poor infrastructure, and limited industrialisation.
A high contribution of the tertiary sector suggests a developed economy.
A decreasing population or low labour supply is not a standard measure of development.
Thus, the correct description is based on low income per head.
Answer:A. the income per head is low