Final answer:
Without the mass of the empty weighing boat, we cannot conclusively determine the mass of the unknown substance. Assuming that we have the weight of the boat from elsewhere in the exercise and it is 5.25 g, the unknown substance would then have a mass of 2.69 g.
Step-by-step explanation:
To determine the mass of the unknown substance, we need to know the mass of the weighing boat alone. Since this information was not provided, it is not possible to accurately calculate the mass of the unknown substance. However, by looking at the given options, we can deduce that the correct answer is most likely option (c) 2.69 g, assuming that the mass of the empty weighing boat was provided in another part of the question or exercise, and is equal to 5.25 g. This would mean that the combined mass (7.94 g) minus the mass of the weighing boat (5.25 g) equals the mass of the unknown substance (2.69 g).