To determine which substance is most likely Marjorie's unknown, we would need more context or specific properties of the unknown, such as its chemical properties, appearance, behavior in reactions, or physical characteristics (like boiling point, melting point, solubility, etc.). Without that information, it is challenging to ascertain which substance (SiO2, FeCl3, C10H8, or NaCl) is Marjorie's unknown.
Please provide additional details or properties of the unknown substance for a more accurate assessment.