None of Beethoven's symphonies are known as Fur Elise.In April 1810, Beethoven composed a short, romantic composition, the Bagatelle in A minor, that became known as "Für Elise".
In 1810, Beethoven composed the Bagatelle in A minor that became known as "Für Elise" for Therese Malfatti. This was the one whom Beethoven was considering marrying at that time, but the marriage...
If you mean what key is Für Elise set in, the piece is predominantly in A minor, but it moves through several keys throughout the piece. Please see the link below for more details.