Where the country's name come from and when the country got this name, "Indonesia"? Was in given in 20th century?
The name itself is far older than the country:
The name Indonesia derives from the Greek words Indós and nèsos, meaning "Indian island". The name dates to the 18th century, far predating the formation of independent Indonesia. In 1850, George Windsor Earl, an English ethnologist, proposed the terms Indunesians — and, his preference, Malayunesians — for the inhabitants of the "Indian Archipelago or Malayan Archipelago"
The country was so named when it declared independence at 10.00 am on Friday, 17 August 1945:
WE THE PEOPLE OF INDONESIA HEREBY DECLARE THE INDEPENDENCE OF INDONESIA.