Statement c is correct: The World Happiness Report ranks 149 countries by how happy their citizens perceive themselves to be. The report has been published by UN Sustainable Development Solution Network 

The rankings are based on polling (Gallup World Poll) which looks at six variables: Gross Domestic Product Per Capita (Purchasing Power Parity), Social Support, Healthy life expectancy at birth, Freedom to make life choices, Generosity, Perceptions of corruption. 

Source: The Hindu