ANS:C - exchange between a liquid phase and an organic resin phase.

The method of purification of leach liquor using ion exchange involves exchange between a liquid phase and an organic resin phase. In ion exchange, ions dissolved in a liquid phase are exchanged for ions of a similar charge present in an organic resin phase. This exchange process occurs at the surface of the resin beads. The ions from the liquid phase adsorb onto the resin, displacing ions originally present on the resin. As a result, the composition of the liquid phase is altered, leading to purification or separation of desired ions from the solution. Therefore, the correct option is: exchange between a liquid phase and an organic resin phase.