The correct answer is - Greek.
Greek is not a language that belongs to the Celtic languages branch, but instead it is a unique language that is in its own separate branch.
The Celtic languages, were very widely dispersed across the European continent, and they were the dominant languages in Britain and Ireland, most of Spain, most of Western Europe, through parts of Central Europe and the Balkans, and even in the central part of Anatolia.
Nowadays, the Celtic languages are kept alive and revived at some places, like Brittany, Wells, Ireland, Isle of Man, Manx, Cornwall, Scotland, and they fall into two branches of Celtic languages, Gaelic Celtic and Breton Celtic.
