The correct answer is A. The reaction released heat
Explanation:
A reaction is considered to the exothermic in the case it releases energy, this includes heat being released during the reaction. Due to this, a key indicator of an exothermic reaction is a change in temperature that would show heat being released. Additionally, an exothermic reaction is the opposite of an endothermic reaction, in which energy is absorbed. According to this, the observation that would help the scientist to conclude a reaction is exothermic is "The reaction released heat".
