The agents is the substance in a redox reaction that donates electrons.
a. buffering
b. half-reaction
c. reducing
d. oxidizing

Heres a handy rhyme:
OIL RIG
Oxidation Is Loss of electrons (donates)
Reduction Is Gain of electrons (accepts/receives) 
In this instance, we are looking for the substance that gets oxidized. The Reducing agent CAUSES reduction, but it itself get oxidized. therefore youre looking at c, the reducing agent  

OILRIG.
oxidation is loss of electron while reduction is the gain. oxidizing agents oxidize to be reduced while reducing agents reduce to be oxidized. reducing agents donate electrons.