(c) 59%
59% of Belgium's population lives in the Flemish region and speaks Dutch.
Source: Belgium and Sri Lanka, Chapter 1
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The passage clearly states: "Of the country's total population, 59 per cent lives in the Flemish region and speaks Dutch language." Note that 40% live in Wallonia (French-speaking) and 80% refers to French-speakers in Brussels — a common distractor in this MCQ.