We are given with the mass of Arsine (
The mass of arsine is 15g
there is a relation between moles, mass and molar mass of any compound which is

The molar mass of Arsine = atomic mass of As + 3X atomic mass of H
the molar mass of Arsine = 74.92 + 3X 1 = 77.92 g/mol
Let us calculate the moles as

I have done this question before and even though you didn’t provide the options. I think the correct option is elements
In order to find the amount of atoms you have to find the moles then multiply by Avogadro's Constant (L)
mol =
=
= 3.73 mol
Number of atoms of Cu = mol × L
= 3.73 mol × (6.02 × 10²³)
= 2.245 × 10²⁴ atoms of copper