In order to ensure safety when using a deep fryer, it is important to do which of the following?

Keeping the kettle filled to the fill line is crucial for ensuring safety when using a deep fryer. This is important because overfilling can cause the oil to spill over when food is added, potentially leading to burns or a fire hazard. The fill line is established to maintain a proper oil level that allows for safe cooking and prevents overflow while ensuring that the food is submerged adequately for even cooking.

While using a thermometer is important for checking the oil temperature, it does not address the risk associated with oil level. Setting the thermostat after filling the fryer could result in oil spillage if the fryer is filled too high, posing safety risks. Checking the oil flavor is part of maintaining food quality, but it does not directly relate to the immediate safety concerns associated with frying. Thus, maintaining the appropriate oil level in the kettle significantly contributes to safety in deep frying operations.