The correct statement about the original cool liquid is:
The liquid is a homogeneous mixture because the substance that formed the large crystals was uniformly dissolved.
This is because a homogeneous mixture has a uniform composition throughout, and before the salt crystal was added, the dissolved substance could have been uniformly mixed with the liquid. The formation of large crystals upon addition of a single salt crystal indicates that the liquid had the capacity to dissolve the salt uniformly before crystallization occurred.