Answer
Ammonia $(NH_3)$ is the richest source of nitrogen on a mass percentage basis.
Work Step by Step
The mass percent of nitrogen can be calculated using this equation:
$$mass \space percentage \space (N) = \frac{molar \space mass (N) \times amount \space of \space N \space in \space 1 \space molecule}{molar \space mass \space (compound)} \times 100\%$$
(a) $ (NH_2)_2CO $ : ( 1.008 $\times$ 4 )+ ( 12.01 $\times$ 1 )+ ( 14.01 $\times$ 2 )+ ( 16.00 $\times$ 1 )= 60.06 g/mol
$$\frac{14.01 \times 2}{60.06} \times 100\%= 46.7 \%$$
(b) $ NH_4NO_3 $ : ( 1.008 $\times$ 4 )+ ( 14.01 $\times$ 2 )+ ( 16.00 $\times$ 3 )= 80.05 g/mol
$$\frac{14.01 \times 2}{80.05} \times 100\%= 35.0 \%$$
(c) $ HNC(NH_2)_2 $ : ( 1.008 $\times$ 5 )+ ( 12.01 $\times$ 1 )+ ( 14.01 $\times$ 3 )= 59.08 g/mol
$$\frac{14.01 \times 3}{59.08} \times 100\%= 71.1 \%$$
(d) $ NH_3 $ : ( 1.008 $\times$ 3 )+ ( 14.01 $\times$ 1 )= 17.03 g/mol
$$\frac{14.01 \times 1}{17.03} \times 100\%= 82.3 \%$$
