Answer: I assume that the answer A explains why they are developed countries and also the most obvious answer.
If people are healthy and educated in a country it is assumed that there is a vast amount of access to basic needs and services (suggesting why A is the answer).
Why the other answers aren't necessarily correct:
Low GDP can be bad for the economy ruling out Answer B
High Trade Balance as in a deficit or a surplus? Are they producing more than they buy or buying more than they produce? A surplus would indicate that the economy is good, but a deficit would be bad for the economy. Depending on this, Answer C may/may not be the answer.
People receiving little health care means that they may pass away as a result of little to no access to basic needs and services. Answer D would be the least correct answer out of the four answers.