3.which of the following signifies a return to music that has been played before?
a. half cadence
b. da capo sign
c. key signature
d. authentic cadence

I am pretty sure that the following which signifies a return to music that has been played before is the second option from the scale represented above : B.Da capo sign. I choose this one because it has meaning "from the beginning'' which nicely coincides with the statement in the question.
Do hope it helps!
 

Your answer has to be c