Final answer:
In an endothermic reaction, the temperature of the solution decreases.
Step-by-step explanation:
The student can determine that the reaction between citric acid and baking soda is endothermic if the temperature of the solution decreases. In an endothermic reaction, energy is absorbed from the surroundings, resulting in a decrease in temperature. The other options mentioned in the question - changes in color, formation of gas bubbles, and increase in temperature - do not indicate an endothermic reaction.