Final answer:
Moles are useful for determining the number of atoms in a 10 g sample of an element by using the molar mass and Avogadro's number.
Step-by-step explanation:
The correct answer to the question 'Which of the following best completes the statement: moles are useful for determining...?' is (A) the number of atoms in a 10 g sample of an element. Moles provide a specific measure of the number of atoms or molecules in a bulk sample of matter because one mole contains Avogadro's number, which is 6.022×1023 discrete entities. Using the concept of molar mass, one can convert the mass of an element to moles and thereby calculate the number of atoms within that sample.