Sodium thiosulfate, na2s2o3, is used as a "fixer" in black and white photography. identify the reducing agent in the reaction of thiosulfate with iodine.2s2o32–(aq) + i2(aq) → s4o62–(aq) + 2i–(aq)

Answer is: reducin agent in this reaction is thiosulfate (S₂O₃²⁻).
Balanced chemical reaction: 2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I⁻(aq).
Reducing agent is element or compound who loose electrons in chemical reaction. Sulfur in thiosulfate change oxidation number from +2 to +5 tetrathionate anion (two sulfur atoms in the ion have oxidation state 0 and two atoms have oxidation state +5).

You know how the first one if filled up like the objects do the same but with the other categories