Answer:
A) Na
Step-by-step explanation:
We analyze the change of electrons in the equation. 
So: 
S (0) --> S (-2) 
The oxidizing agent is the one that gains electrons. Sulfur goes from 0 to -2 then it is gaining electrons. 
Na (0) --> Na (+1) 
The reducing agent is the one that loses electrons. Sodium goes from 0 to +1 which means that it loses electrons. 
Therefore the reducing agent is Na.