(D) Switzerland
Switzerland is the best example of a 'coming together' federation, where independent states voluntarily joined to form a federation for mutual benefit.
There are two types of federations: 'coming together' (independent states voluntarily unite — e.g., USA, Switzerland, Australia) and 'holding together' (a large country divides power among states — e.g., India, Spain, Belgium). India is a classic example of a 'holding together' federation, so option (C) is wrong. Switzerland fits 'coming together' as historically independent cantons voluntarily united.