Final answer:
Not pre-measuring the mass of the second evaporating dish will lead to decreased accuracy in the experimental results, as it is necessary for calculating the mass of the residue.
Step-by-step explanation:
If a student forgets to pre-measure the mass of the second evaporating dish, it will likely lead to decreased accuracy in the results of the experiment (option c). The mass of the evaporating dish is crucial for determining the mass of the residue after evaporation. Without knowing the mass of the dish beforehand, the student cannot accurately calculate the mass of the residue alone. This would impact any calculations based on the mass of the product or reactant in the dish. Therefore, measurements taken both before and after a chemical reaction or physical process are critical for ensuring precise and accurate results.