Final answer:
Correct option: a chemical reaction, because a change in temperature can be evidence that a chemical reaction has taken place. The beaker's increase in temperature upon mixing two chemicals is indicative of a chemical reaction, which is characterized by the formation of new substances and often involves energy changes such as heat release.
Step-by-step explanation:
The beaker becoming warm upon mixing two chemicals suggests that a chemical reaction has likely occurred. During a chemical reaction, new substances are formed, and these processes often involve energy changes, such as the release or absorption of heat. An increase in temperature is a common sign of an exothermic reaction, where energy is released into the surroundings, indicating a chemical change rather than a physical one. Arya's observation does not merely show a change in state (like melting or boiling) or a mixture where the identities of the substances remain the same. Instead, the production of heat suggests that the molecular structures of the starting materials have been altered to form new chemical entities.