The correct option is C (NH4)2Cr2O7⟶N2+Cr2O3+4H2O
Disproportionation reaction is a specific type of redox reaction in which a species is simultaneously reduced and oxidised to form two different products.
In the option A: Cl2 (0) becomes Cl− (-1) and ClO− (+1).
In the option B: H2O2 (oxidation state of O = -1) becomes H2O (-2) and O2 (0).
In the option C: Cu (+1) becomes Cu (+2) and Cu (0).
In the option D: (NH4)Cr2O7 (oxidation state of Cr = +6) becomes only Cr2O3 (+3). There is no oxidized product of Cr.
Hence, option D is the correct answer.