There are a couple of ways to approach this problem, but one common method is to count the number of nitrogen atoms in each molecule of NH₄, and then multiply that by the total number of NH₄ molecules in the compound.
Each NH₄ molecule contains one nitrogen atom, so 4(NH₄)₂ contains a total of 4 x 2 = 8 NH₄ molecules.
Therefore, the total number of nitrogen atoms in the compound is:
8 NH₄ x 1 N per NH₄ = 8 N atoms
So the answer is (C) 8.