If the student's unknown is a white solid and its density was calculated to be about 1.7 g/mL, it is most likely C. phosphoric acid.
Phosphoric acid has a density of 1.685 g/mL and can appear as a white solid or a colorless liquid, fitting the description given. Sodium hydroxide, while it is also a white solid, has a much higher density of 2.13 g/mL, which does not match the calculated density.