What is the main purpose of treating phosphate rock with acids in fertiliser production?

The correct option is To make it soluble. This answer is correct because it matches the approved learning objective to recall that phosphate rock is obtained by mining but cannot be used directly as a fertiliser. (Chemistry only) in the subtopic Production and uses of NPK fertilisers.

The correct option is To make it soluble. To make it soluble is correct because it directly supports the approved learning objective to recall that phosphate rock is obtained by mining but cannot be used directly as a fertiliser. (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.