The mobile phase in paper chromatography is usually:

Explanation

The correct answer is a non-polar organic liquid. In paper chromatography, the mobile phase is typically a non-polar organic solvent that moves up the paper, carrying the different components of the mixture with it. This helps separate the components based on their affinity to the stationary phase, which is usually water (a polar substance) within the paper. This polarity difference is crucial for separation. Water is incorrect as it serves as the stationary phase. Sulphuric acid and silver nitrate are not used as mobile phases in this technique due to their inappropriate properties for chromatography.