The quality of a population is primarily determined by its literacy rate. Literacy refers to the ability to read, write, and use arithmetic skills, which are fundamental for personal development, economic growth, and social progress. A higher literacy rate indicates that more individuals in the population have access to education, which enhances their cognitive abilities, critical thinking, and overall quality of life.
Here's why the other options are incorrect:
- Option (1): Literacy rate: This is the correct answer, as literacy is a key indicator of population quality. Educated individuals are better equipped to contribute to society, make informed decisions, and improve their living standards.
- Option (2): Illiteracy: Illiteracy refers to the lack of reading, writing, and basic arithmetic skills. While it is important to reduce illiteracy, the focus should be on increasing literacy rates, which directly improves population quality.
- Option (3): Skill formation: Skill formation is crucial for economic development and employment opportunities. However, it is a subset of education and builds upon literacy. Therefore, literacy is the foundational factor.
- Option (4): Health of a person: Good health is essential for a high-quality population, but it is not the primary determinant. While health is closely linked to literacy (as educated individuals tend to have better health awareness), literacy itself is a more comprehensive measure of population quality.
Answer: $\boxed{\text{Literacy rate}}$