The compound which undergoes disproportionation in alkaline solution is :

Disproportionation reaction is that reaction in which one compound undergoes both reduction and oxidation processes.

$\stackrel{+6}{{\mathrm{Cl}}_2{\mathrm{O}}_6} + 2\mathrm{NaOH} \to \stackrel{+7}{{\mathrm{NaClO}}_4 } + \stackrel{+5}{{\mathrm{NaClO}}_3} + {\mathrm{H}}_2\mathrm{O}$

Cl is oxidized from +6 to +7 when Cl₂O₆ is converted into NaClO₄ and Cl is reduced from +6 to +5, when Cl₂O₆ is converted into NaClO₃.