An example of a disproportionation reaction is

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Disproportionation = same element simultaneously oxidised and reduced.

Step 1 — Check option (3): 
- Cu in :  (reactant)
- Cu in :  → oxidised
- Cu in :  → reduced
- Same element (Cu) both oxidised and reduced → Disproportionation ✓

Step 2 — Eliminate others:

(1) :
- Mn:  (reduced) and  (reduced)
- O:  (oxidised in )
Different elements change → this is a decomposition/redox but Mn only reduces; O is oxidised. Not a pure disproportionation.

(2) Two different elements (Mn reduced, I oxidised)

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