Step 1: Understand the concept

Number of atoms is directly proportional to the number of moles. For the same mass, element with the lowest molar mass will have the highest number of atoms.

Step 2: Compare approximate molar masses

Pb ≈ 207 g mol⁻¹
Po ≈ 209 g mol⁻¹
Pt ≈ 195 g mol⁻¹
Pr ≈ 141 g mol⁻¹

Step 3: Identify the smallest molar mass

Praseodymium (Pr) has the lowest molar mass among the given elements.

Step 4: Final conclusion

Hence, 10⁻⁹ g of Pr will contain the highest number of atoms.