Step 1: Recall the two ways federations form.
Some federations form when independent states voluntarily join together for shared benefits like defence and trade, this is called coming together. Others form when one large country decides to divide its power to keep diverse regions united, this is called holding together.
Step 2: Sort the given countries.
India, Belgium, and Spain each started as one country and later shared power with their regions to manage internal diversity, so they are holding together federations.
Step 3: Check Australia's history.
Australia was originally six separate British colonies that chose to unite in 1901 to form the Commonwealth of Australia, keeping some powers for themselves while giving others to the centre. That makes it a coming together federation.
Step 4: State the final answer.
Hence, the correct answer is option (D): Australia.