Which mixture of the solutions will lead to the formation of...

The common ion between the colloidal particles and the remaining solution gets adsorbed on the surface of the colloidal particles and thus the particles acquire charge.

Formation of negatively charged colloidal $[ AgI ] {I}^{-}$ sol:

50ml of $1M AgN{O}_{3} + 50ml2.0\text{M}KI$

Millimoles of \left[A{g}^{+}\right] = 50 \times 1m = 50mmoles$</p><p>Millimoles of \left[{I}^{-}\right] = 50 \times 2 = 100mmoles$

${n}_{\left({I}^{-}\right)} > {n}_{A{g}^{+}}$

$∴$ it will form negatively charged solution.
$[ AgI / {I}^{-} ]$