Answer:
Ammonia is the richest source of nitrogen on a mass percentage basis because it has 82.35% of nitrogen by mass.
Explanation:
Percentage of element in compound :

(a) Urea, 
Molar mass of urea = 60 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 2

(b) Ammonium nitrate, 
Molar mass of ammonium nitrate = 80 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 2

(c) Nitric oxide, NO
Molar mass of nitric oxide = 30 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 1

(d) Ammonia, 
Molar mass of ammona = 17 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 1
Ammonia is the richest source of nitrogen on a mass percentage basis because it has 82.35% of nitrogen by mass.
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It seems that you have missed to attach the given map and options for us to answer this question, so I had to look for it. Anyway, here is the answer. The statement that best describes the whether shown by the purple combination of semicircles and triangles on a line on a weather map is a <span>cold front can cause heavy rain, thunder, and lightning. Hope this helps.</span>