a) According to the text, people feel happy because 3. they are healthy.
b)1. People in Bhutan feel happy because healthcare and economy are slowly improving and because of the mountains which offer good conditions for a healthy lifestyle.
2. People in Iceland are happy because of the clean air and the fact that they have enough doctors to take care of them.
3. People in Denmark are happy because they are generally content with theit life, feel safe in their country and earn enough money to keep them financially secure.
4. The king of Bhutan believed that happiness is the best way to determine how much a country has evolved over time.
c. various - many; to strengthen - to improve; to contain - to equal; the same - another (different thing but the same idea)