The correct answer is A. Amide.
Urea is a chemical compound with the formula (NH2)2CO. It is a white, odorless solid that is widely used as a fertilizer. Urea is also produced in the body as a waste product of protein metabolism.
The nitrogen in urea is in the form of an amide group. An amide group is a functional group that consists of a nitrogen atom bonded to a carbonyl group. The amide group is a stable group that is not easily broken down. This makes urea a good source of nitrogen for plants.
The other options are incorrect because they represent forms of nitrogen that are not found in urea. Ammoniacal nitrogen is nitrogen that is in the form of ammonia (NH3). Nitrate nitrogen is nitrogen that is in the form of nitrate (NO3-). Nitrite nitrogen is nitrogen that is in the form of nitrite (NO2-). These forms of nitrogen are not found in urea because urea is a non-ionic compound.
In conclusion, the correct answer is A. Amide.