Question

H3AsO3 + NO3- + H+ ---> HNO2 + H3AsO4
In the above redox reaction, use oxidation numbers to identify the element oxidized, the element reduced, the oxidizing agent and the reducing agent.

Answer 1

Here are the rules for assigning oxidation states.
https://www.chemteam.info/Redox/Redox-Rules.html

Here is how you do it following those rules. Study this and you will be able to do it yourself.

H3AsO4. O is -2 each for a total of -8. H is +1 each for a total of +3. That leaves As. Since every eompound is zero, you know As must be +5 BECAUSE +3 +5 -8 = 0

The element oxidized is the one that lost electrons.
The element reduced is the one that gained electrons.
The oxidizing agent is the one that is reduced.
The reducing agent is the one that is oxidized.

I'll be happy to check your work. Look at N for the other element that changes oxidation state.