I believe road access was emphasised and Turkmenistan has a doughnut-shaped population distribution (the major population centres tend to be near the borders of other countries). I found the source of the quiz here. To be honest, in quiz format it's all rather arbitrary and isolation is not exactly quantifiable. This is one of my earlier quizzes (nearly seven years old), and it is both something I would avoid making today but also the data may be outdated now.