Consider the following transformation :
$2{\mathrm{CuX}}_2\stackrel{\text{ Room temperature }}{⟶}2\mathrm{CuX}+{\mathrm{X}}_2\uparrow$
Then X^- can be :

X should have a small size so that it can readily decompose into CuX and X₂ at room temperature. Among the given options, CN^- and I^- can be X because Cu(CN)₂ and Cul₂ are unstable and readily decomposes into CuCN and CuI respectively at room temperature.

$\begin{array}{ccc}2{\mathrm{CuCN}}_2& \stackrel{\mathrm{room} \mathrm{temperature}}{\to }& 2\mathrm{CuCN}+{\mathrm{CN}}_2\\ 2{\mathrm{CuI}}_2& \stackrel{\mathrm{room} \mathrm{temperature}}{\to }& 2\mathrm{CuI}+{\mathrm{I}}_2\end{array}$