B. 1, 2, and 3 only
Observations 1, 2, and 3 support the conclusion that a chemical reaction took place. The color change indicates a chemical change, the volume change indicates a reaction occurred, and the temperature change indicates an energy change, which is typical of a chemical reaction. Observation 4 is also relevant as it provides additional information about the combined liquids, but it is not necessary to support the conclusion of a chemical reaction taking place.