Surat Saba verse 13

Allaah says “And few of My servants are grateful.” (Quran 34:14)

In this ayat when Allaah uses the word “servants”, is Allaah referring to:

(a) the believers (few amongst the believers are grateful)

or

(b) both the Muslims and the Kuffar (few amongst humanity/jinn are grateful regardless of their religious belief)?