If the student's unknown is a white solid with a density of about 1.7 g/mL, it is most likely C. phosphoric acid.
Sodium hydroxide has a higher density (2.13 g/mL) and is also a white solid, but it doesn't match the density of the unknown. Phosphoric acid has a density (1.685 g/mL) that is close to the stated density of 1.7 g/mL and can be either a white solid or a colorless liquid, making it the most appropriate choice based on the provided data.