In electric discharge machining (EDM), the process involves using electrical energy to remove material from a workpiece. The material removal occurs by a series of repeated electrical discharges between the workpiece and an electrode, which is typically made of a conductive material such as copper or graphite. These discharges vaporize and erode the material, resulting in the desired shape or form.

To achieve a better surface finish in EDM, the frequency and discharge current are important parameters that need to be considered. The correct answer to the question is option 'C', which states that a better surface finish is obtained at high frequency and low discharge current.

Let's understand why this combination is preferred for achieving a better surface finish in EDM:

1. High Frequency:
- High frequency refers to the number of electrical discharges that occur per unit time. In EDM, a higher frequency means more discharges, which results in smaller craters on the workpiece surface.
- Smaller craters lead to a smoother surface finish because the material is removed in smaller increments, reducing the roughness and irregularities on the workpiece.
- A high frequency also allows for faster material removal, which can contribute to a better surface finish by minimizing the time for heat accumulation and reducing the chances of thermal damage to the workpiece.

2. Low Discharge Current:
- Discharge current refers to the amount of electrical current flowing between the workpiece and the electrode during the EDM process.
- A low discharge current helps to reduce the heat generated during the discharges. Excessive heat can lead to thermal damage, such as material melting or recast layer formation, which can deteriorate the surface finish.
- By keeping the discharge current low, the energy input is controlled, resulting in a more controlled and precise material removal process.
- Additionally, a low discharge current helps to minimize the electrode wear, which can also affect the surface finish. Less electrode wear means that the electrode maintains its shape and geometry, resulting in more accurate machining and a smoother surface finish.

In summary, a high frequency and low discharge current combination in electric discharge machining allows for more controlled material removal, reduces heat generation, minimizes electrode wear, and ultimately leads to a better surface finish on the workpiece.