The question is about critical relative humidity, or CRH, which is the air humidity level at which a fertilizer begins soaking up moisture. The fertilizer with the lowest CRH is the one that turns hygroscopic first, so it is the most hygroscopic of the group.
- Ammonium sulphate: CRH is high, roughly 79 to 81 percent, so it needs quite humid air before it starts absorbing moisture.
- Urea ammonium sulphate: as a blended fertilizer its CRH is pulled down somewhat compared to the pure salts, but it still stays well above the value for ammonium nitrate.
- Ammonium nitrate: CRH is low, in the range of about 59 to 65 percent, meaning it starts drawing moisture out of the air even at moderate humidity. This is why ammonium nitrate is notoriously prone to caking and is handled with extra care in storage.
- Ammonium chloride: CRH is close to 79 percent, similar to ammonium sulphate, so it is also fairly stable in typical storage conditions.
Ranking all four by CRH, ammonium nitrate sits clearly lowest, which makes it the fertilizer that turns hygroscopic at the mildest humidity level.
Let's summarize:
- A lower CRH means a fertilizer absorbs moisture more easily.
- Ammonium nitrate has the lowest CRH of the four options, making it the most hygroscopic.
So the correct answer is ammonium nitrate, option 3.