Definition of a Coming Together Federation:
A coming together federation is a type of federal system where previously independent states or regions join together to form a single political entity, sharing certain powers and responsibilities while retaining a significant degree of autonomy. This form of federation is characterized by a voluntary association of states or regions, with each member having equal representation and sharing decision-making powers.

Explanation of the Correct Answer:
The correct answer is option 'D': Switzerland, USA, and Australia. Here's an explanation of why these countries are examples of coming together federations:

1. Switzerland:
Switzerland is an example of a coming together federation. It is made up of 26 cantons, which were historically independent states or territories that voluntarily joined together to form the Swiss Confederation. Each canton has its own constitution, government, and legislative authority, while the federal government handles matters of national importance.

2. USA (United States of America):
The United States of America is another example of a coming together federation. It was formed by the original thirteen colonies, which were independent entities before the American Revolution. These colonies voluntarily joined together to form a federal republic, where power is divided between the federal government and individual states.

3. Australia:
Australia is also a coming together federation. It consists of six states and two territories, each with its own government and constitution. The states and territories voluntarily agreed to form the Commonwealth of Australia in 1901, with certain powers delegated to the federal government while retaining significant autonomy.

Other Options:
The other options listed in the question are not examples of coming together federations:

- Belgium: Belgium is an example of a holding together federation, where previously independent regions agree to unite and form a single country while granting certain powers to the central government. It is not a coming together federation.
- India: India is a holding together federation, similar to Belgium. It consists of states and union territories that voluntarily joined together to form the Republic of India. It is not a coming together federation.
- Sri Lanka: Sri Lanka is not an example of a coming together federation. It is a unitary state where power is centralized in the national government and there is no significant autonomy granted to individual regions.

Conclusion:
In conclusion, the correct answer is option 'D': Switzerland, USA, and Australia. These countries are examples of coming together federations, where previously independent states or territories voluntarily joined together to form a single political entity while retaining a significant degree of autonomy.