The thermal energy is where the work of friction comes from. That is what stops it eventually. In this case a counter force of 10N is applied over the distance of 30.0m. The energy is given by Force*Distance. Here this is 300J. This friction work is the thermal energy.
A 20.0 kg curling stone travels 30.0 m along the ice surface. If the frictional force is 10.0 N, the thermal energy produced is