Option A is correct: The lines "Currently, the global average for nitrogen use efficiency is approximately 0.4, meaning 40 per cent of the total nitrogen added to cropland goes into the harvested crop while 60 per cent is lost to the environment, says a study....Plants take the nitrogen they need to grow, and the excess is left in the ground, water and air.” and “This results in significant emissions of nitrous oxide...and other forms of nitrogen pollution, contamination of river water.” highlight the need of nitrogen for plant growth and the result of unused nitrogen on water and air. Thus, improving the nitrogen use efficiency can not only lead to better production but also to reduced pollution.

Option B is incorrect: While the passage mentions, “more than half of the world's population is nourished by food grown with fertilizers containing synthetic nitrogen”, it does not state that the production of synthetic nitrogen fertilizers cannot be stopped.

Option C is incorrect: The question is asking us to identify the most logical, rational and crucial message implied by the passage, rather than an inference or solution. The passage only talks about nitrogen use efficiency and the impact of excess nitrogen on the environment. It does not go beyond this to suggest that we must identify and cultivate crops that require excess nitrogen.

Option D is incorrect: These alternatives may have their merits, but the passage only highlights the environmental consequences of excessive nitrogen use and pollution caused by synthetic nitrogen fertilizers. It does not explicitly advocate replacing conventional agriculture using synthetic fertilizers with specific alternatives such as agroforestry, agroecosystems, and organic farming.