Phosphate rock cannot be used directly as a fertiliser because it is:

The correct option is Insoluble. This answer is correct because it matches the approved learning objective to describe treatment of phosphate rock with sulfuric acid to produce soluble salts for fertilisers. (Chemistry only) in the subtopic Production and uses of NPK fertilisers.

The correct option is Insoluble. Insoluble is correct because it directly supports the approved learning objective to describe treatment of phosphate rock with sulfuric acid to produce soluble salts for fertilisers. (Chemistry only). This belongs to the subtopic Production and uses of NPK fertilisers within The Haber process and the use of NPK fertilisers, so the explanation must stay tied to that curriculum context. The other options are incorrect because they either do not answer this learning objective, use a vague statement, or move away from Production and uses of NPK fertilisers.