ANS:D - all 'a', 'b' & 'c'.

The baffles or horizontal compartmental plates provided in mechanically agitated liquid-liquid extractors serve multiple purposes:

1. Reducing axial mixing: Baffles or plates help in reducing the axial mixing of the two phases by directing the flow of liquids and preventing them from flowing directly from top to bottom or vice versa. This ensures better contact between the two phases, enhancing mass transfer efficiency.
2. Increasing the rate of extraction: By promoting better mixing and contact between the two phases, baffles or plates can increase the rate of extraction. This is because they help to expose more surface area of the dispersed phase to the continuous phase, facilitating mass transfer of solutes between the phases.
3. Maintaining the concentration difference between the two phases: Baffles or plates can help in maintaining a concentration gradient between the two phases, which is essential for efficient mass transfer and extraction. By preventing excessive mixing, they help to preserve the concentration difference, ensuring that the driving force for mass transfer remains high.

Therefore, the correct answer is: all 'a', 'b' & 'c': Baffles or horizontal compartmental plates in mechanically agitated liquid-liquid extractors contribute to reducing axial mixing, increasing the rate of extraction, and maintaining the concentration difference between the two phases.