South Korea
Correct Answer: Iceland

A brief explanation why…

According to 2016 numbers hosted on Internet Live Stats, Iceland has a greater Internet penetration rate than any other country at 100%, with Faeroe Islands, Norway, Bermuda and Andorra rounding out the top five most-connected countries, having more than 96% of their citizens online.

Unsurprisingly, the top of the list is comprised of fairly small countries by population count, with Norway being the most populated at more than five million people, Iceland counts 331,778 people, and the other nations have populations well under 100,000, making it fairly easy to ensure they're all connected.

The UK didn't make the site's top 10 list, with the country's 60 million people being 92.6% connected, while South Korea and Australia are side-by-side with penetrations of 85.7% and 85.1% for their headcounts of 43 million and 20 million, and Japan's 115 million are 91.1% connected. It's estimated that 88.5% of the 286 million people in the US have Internet, putting it around 20th on the list.