Answer: The correct answer is dependent variable.
Step-by-step explanation:
Independent variable is defined as the variable which remain as such and has no effect on the change of another variable. For Example: Time
Dependent variable is defined as the variable whose value changes with respect to any other variable.
Constant variable is defined as the variable whose value cannot be change if any value is assigned to it.
According to the question:
Three boxes of different materials having same size is kept in the Sunlight to check the warmth of air inside the box.
Thus, sunlight is independent variable and warmth of air inside the box is the dependent one because if the sunlight falling on the boxes increases, the temperature of the air also increases and vice-versa.
Hence, the correct answer is dependent variable.