Consider the following unbalanced redox reaction:
mno4-(aq) + sbh3 (aq) > mno2(s) + sb(s)

what is the oxidizing agent in the reaction?
select one:
a. mno4-
b. sbh3
c. mno2
d. sb

The oxidizing agent is the one that is reduced, this is the one that gains electrons and so its oxidation number decreases (it becomes more negative or less possitive).

Now establish the oxidation number of each element:

In the MnO4 - ion: Mn 7+    O 2-
In the SbH3: Sb 3-    H 1+
In the MnO2:  Mn 4+    O 2-
Sb: 0

Then Mn is reduced from Mn 7+ to Mn 4+ which means that it is the oxidizing agent.