Hello!
To do this, use the molar mass. This is how much a mole of an atom weighs. A mole is 6.02214076×10²³ atoms.
Molar masses of:
Se: 78.96 g/mol
Cu: 63.546 g/mol
Ba: 137.327 g/mol
Now, the element with the highest molar mass will have the fewest atoms. This is because the element weighs more, so therefore for the same amount of mass, there will be less of the element needed to reach that mass.
Therefore, 10g of Ba would have the fewest number of atoms.
Hope this helps!
