Final answer:
The representative particle for a molecule is a molecule, and substances exist naturally as atoms, ions, or molecules depending on their chemical composition. Avogadro's number indicates the quantity of these particles in one mole.
Step-by-step explanation:
The representative particle for a molecule is a) molecule. A representative particle is the smallest unit in which a substance naturally exists, which can be atoms, ions, or molecules. For molecular compounds like water (H₂O) and carbon dioxide (CO₂), the representative particle is the molecule.
The representative particle for ionic compounds such as sodium chloride (NaCl) and calcium nitrate (Ca(NO₃)₂) is the formula unit. Avogadro's number, which is 6.02 × 10²³, represents the number of representative particles in a mole of a substance.