Are you using the term “mysteries” correctly?

Does “mysteries” in theology refer to:

a.  the sacraments
b. articles of faith which can’t be proven by pure reason (Vatican 1)
c. things which were unknown in the previous dispensation


Bavinck answers, “But this mystery is so called, not because it is still hidden in the present, but because it had been unknown n the past.  Now–of all things–it has been made public by the gospel of Christ (Rom. 16:25, Col. 1:26; 1 Cor 4:1 et al) and from now on will be increasingly manifest in history” (I: 620).