According to the World Bank’s Ease of Doing Business Index 2015, Singapore was ranked the number one country in the world for ease of doing business. The country has consistently maintained top positions in global rankings due to:
- Transparent and efficient regulations,
- Strong legal and financial systems,
- Excellent infrastructure,
- Business-friendly tax policies.
Other options:
- (A) U.S.A: Ranked lower due to regulatory complexity.
- (B) New Zealand: Was close but ranked second in 2015.
- (D) Hong Kong: Also in the top rankings, but not number one in 2015.
- (E) India: Ranked much lower in 2015.
Thus, the correct choice is:
\[
\boxed{\text{Singapore}}
\]