The population of the ground squirrels will be limited by the population of the hawks. The population of the hawks, thus, will be a limiting factor for the population of the ground squirrels. The Hawks are the birds of prey that feeds on squirrels. With the increased number of hawks in the same area as them will put these squirrels in danger. The population of the tree squirrels, however, will have no affect on the population of the ground squirrels. The genetic variability and breeding season are not limiting factors for their population either.
Hence, the correct answer is 'option B - a population of hawks'.