The effectiveness of Rock phosphate increases in soil

Correct option is C

Rock phosphate is a natural phosphatic fertilizer with low water solubility.
Its solubility increases under acidic soil conditions due to higher H⁺ activity.
Acidic soils enhance dissolution of calcium phosphate present in rock phosphate.
In alkaline and neutral soils, phosphorus remains largely unavailable.
Hence crop response to rock phosphate is better in acidic soils.
Therefore, acidic soil is the correct answer.