Based on the Human Development Index (HDI), which categorizes countries into different levels of development, a country is generally considered highly developed if its HDI is above 0.90.
In the options provided:
- Country A: 0.91 (highly developed)
- Country B: 0.64 (less 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.