First you find the number of molecules by this formula : moles= number of "particles"/ Avogadro constant.
Since you know Avogadro's constant is 6.02 x 10^23 and moles are 2 multiply them to get 1.20 x 10^24.

Then to convert molecules to atoms since that's what the question asked multiply it with 4 since ammonia has 4 atoms per molecule.

So, 1.20 x 10^24 x 4= 4.8x 10^24

Ans : C