which of the following is the richest source of ammonia on a mass percentage basis:

a.NH4NO3

b.NH2CONH2

c.NH3

d.NHC(NH2)2

I think your question is which is the richest source of Nitrogen (not ammonia) as ammonia is in the option itself so ammonia would become 100 % source of ammonia.

Mas percent of Nitrogen is calculated as

= (mass of Nitrogen atom per mass of whole molecule) X 100 %

Mass of one atom of Nitrogen is 14 amu. Dividing it with the mass of given compound. For example ammonia NH₃ has mass 14 + 1 + 1 + 1 = 17  amu

So its mass percent = (14/17) X 100  = 82.24 %

For other compounds :

a. Ammonium Nitrate, NH4NO3 - 34.9 %

b.Urea, (NH2)2CO - 46.64%

d. Guanidine, HNC(NH2)2 - 71.13 %

the richest source of Nitrogen is NH3 (Option 3)