To measure the temperature of food, completely insert the stem of the ________ into the center of the thickest part of the food item.

The correct choice is the thermometer because it is specifically designed to measure temperature accurately. When determining the doneness of various foods, especially meats, ensuring that the thermometer's stem is inserted into the thickest part is crucial for getting an accurate reading. This helps in preventing undercooking, which can pose health risks, as well as overcooking, which may affect the food's quality.

Using a thermometer is a critical practice in culinary settings to ensure food safety and achieve desired cooking results. The other options, such as a fork, spoon, or knife, do not have the capability to accurately measure temperature. They may help with handling or serving food but lack the precision and functionality required for temperature assessment.