Based on the Human Development Index (HDI), a country is generally considered "highly developed" if its HDI score is close to 1.0.
Among the options provided:
- Country A: 0.91 (Highly developed)
- Country B: 0.64 (Moderately developed)
- Country C: 0.39 (Low development)
- Country D: 0.75 (Moderately developed)
Therefore, Country A with an HDI of 0.91 can be considered highly developed.