According to the data released by the Union Finance Ministry for the year 2020-21, Singapore ranks first in terms of ODI (Outward Direct Investment). ODI refers to investments made by a country in other countries, indicating their interest in expanding their businesses or acquiring assets abroad.

Option 1: The United States (USA) is a major global player in terms of investment, but as per the given data, it does not rank first in ODI for the year 2020-21.

Option 3: Mauritius is a popular destination for foreign investment due to its favorable tax policies, but it does not rank first in ODI according to the provided data.

Option 4: The United Kingdom has a significant presence in global investment, but it is not ranked first in terms of ODI based on the given information.

Therefore, the correct answer is option 2: Singapore, as it is stated in the data provided.