Final answer:
The greatest mass of oxygen at STP is measured in grams.
Step-by-step explanation:
The greatest mass of oxygen at STP is measured in grams (option 3). At STP, the molar volume of a gas is 22.4 liters per mole, regardless of the molar mass of the gas. Since the question asks for the quantity of oxygen at STP with the greatest mass, the unit of measurement that corresponds to mass is grams.