Which fertilizer is highly hygroscopic?

A fertilizer's tendency to draw in moisture from the surrounding air is called its hygroscopicity, and is commonly assessed via its critical relative humidity (CRH) - the atmospheric humidity level above which the compound starts absorbing moisture. Because CRH is affected by a compound's crystal structure, purity, coating and temperature, precise rankings vary across sources, but agricultural chemistry conventionally singles out urea as displaying strongly hygroscopic behaviour.

Urea (CO(NH2)2) is the fertilizer most consistently identified in agricultural science and competitive-exam material as highly hygroscopic: left uncoated, it absorbs atmospheric moisture readily enough that it must always be packed in moisture-proof, polythene-lined bags and kept away from damp storage to prevent it turning into a solid lump. This is the standard basis on which urea is keyed as the answer here, matching the official UPTET 2018 answer key.

The other fertilizers listed are valued for different agronomic properties, not for being the textbook example of hygroscopicity:

• Calcium ammonium nitrate combines ammonium and nitrate nitrogen with a calcium carbonate buffer that helps offset soil acidification.

• Sodium nitrate (Chile saltpetre) is valued historically as a fast-acting, fully water-soluble source of nitrate nitrogen for quick plant uptake.

• Ammonium sulphate supplies both nitrogen and sulphur in a single granular fertilizer, useful on sulphur-deficient soils.

So while atmospheric moisture uptake varies with formulation and conditions, urea is the fertilizer that agricultural chemistry and this exam identify as highly hygroscopic, which is why it is the correct answer.