Asolid is added to a liquid in a flask.

what evidence best indicates that an endothermic reaction has taken place?

a. the mixture's color changes.
b. the mixture's temperature decreases.
c. the solid dissolves after it is added to the liquid.
d. the solid and liquid form a heterogeneous mixture.

C. the solid dissolves after it is added to the liquid

Explanation:

Why, you may ask?  Because endothermic reactions absorb energy by undergoing change. And that solid, just absorbed energy.

When something melts, the molecules inside the substance receive more and more energy, until the substance's molecules have enough energy to vibrate and wiggle past each other, breaking their structure. if the solid is receiving more energy from its surroundings, aka the liquid, the reaction is in fact absorbing energy. And if a reaction is absorbing energy, it's an endothermic reaction.