Here's a problem to solve: A student concludes that a chemical reaction performed in a classroom investigation was exothermic. To communicate the validity of this conclusion, which would make the best visual evidence in a presentation?

Among the 4 following options, which is the correct answer?

- A: photographs of reactants and products
 
- B: listing of masses of reactants and products
 
- C: listing of time from start to finish of reaction
 
- D: photograph of a flame produced when the chemicals reacted
 