For the reactants,
For the products,
Here, arsenic (+5 to +3) and carbon (+3 to +4) are the only oxidation numbers changing.
Note that an increase in oxidation number means electrons are lost. Thus oxidation is occurring, and a decrease in oxidation number means electrons are being gained, and thus reduction is occurring.
Also, the compound that contains the element being oxidized is the reducing agent, and the compound that contains the element being reduced is the oxidizing agent.
So, the answers are:
name of the element oxidized: Carbon
name of the element reduced: Arsenic
formula of the oxidizing agent: [tex]\text{H}_{3}\text{AsO}_{4}[/tex]
formula of the reducing agent: [tex]\text{H}_{2}\text{C}_{2}\text{O}_{4}[/tex]