ANS:D - all (a), (b) & (c)

The correct completion for the statement is: "increased biuret formation, high corrosion rate, increased cost of equipment." Excess ammonia, higher pressure, and higher temperature can indeed increase the yield of urea in the ammonia synthesis process. However, there are practical limitations and drawbacks to implementing these conditions:

1. Increased biuret formation: Biuret is an impurity that can form during the urea synthesis process. Excessive ammonia and high temperatures can lead to the formation of biuret, which reduces the quality of the urea product and may have adverse effects on its performance as a fertilizer.
2. High corrosion rate: The conditions required for increasing the yield of urea, such as higher pressure and temperature, can accelerate the corrosion of equipment used in the production process. This corrosion can lead to increased maintenance costs, downtime, and potential safety hazards.
3. Increased cost of equipment: Implementing conditions of excess ammonia, higher pressure, and higher temperature often requires specialized equipment capable of withstanding these conditions. This equipment can be more expensive to purchase, operate, and maintain compared to standard equipment used in urea production.

Therefore, while these conditions may theoretically enhance urea yield, the practical challenges associated with increased biuret formation, high corrosion rates, and the higher cost of equipment make it impractical to implement them extensively in urea production processes.