1. **Stating the problem:** We need to identify the type of variable that a function assigns to exactly one value of the dependent variable. 2. **Understanding the concept of a function:** A function is a rule that assigns each value of the independent variable to exactly one value of the dependent variable. 3. **Explanation:** - The **independent variable** is the input or cause. - The **dependent variable** is the output or effect. 4. **Answer:** The blank should be filled with **independent**. **Final answer:** A function is a rule that assigns each value of the **independent** variable to exactly one value of the dependent variable.