Final answer:
The molecular mass of water refers to the mass of one mole of water molecules (H₂O), which is equal to 18 grams. Therefore, the correct answer is option b. 1 gram molecule of water.
Step-by-step explanation:
The molecular mass of water refers to the mass of one mole of water molecules (H₂O). One mole of water molecules is equal to 18 grams (16 grams of oxygen and 1 gram of hydrogen). This figure is called the molar mass, which is typically given in grams per mole (g/mol). Therefore, the correct answer is option b. 1 gram molecule of water.