The noun (Substantiv/Nomen) Sonstige is feminine in German. Therefore, the correct definite article in the nominative singular is die. So, you say: die Sonstige.
(other)
Alle anderen oder übrigen Dinge, Personen oder Sachverhalte; weitere.
All other or remaining things, persons or facts; further.
Sonstige Fragen werden am Ende der Sitzung behandelt.
Other questions will be addressed at the end of the session.