Explanation:

When a slide is stained for microscopic examination, sometimes there may be an extra stain left behind on the slide. This extra stain can interfere with the observation and interpretation of the slide. To remove this extra stain, the slide can be soaked with filter paper.

Soaking with Filter Paper:
Soaking the slide with filter paper is an effective method to remove the extra stain. This process involves placing a piece of filter paper on the stained area of the slide and allowing it to absorb the excess stain. The filter paper acts as a wick, drawing the stain away from the slide surface.

Steps to remove the extra stain:
1. Take a clean filter paper and cut it into small pieces.
2. Place one piece of filter paper on the stained area of the slide.
3. Gently press the filter paper against the stain, allowing it to absorb the excess stain.
4. Remove the filter paper and examine the slide to see if the extra stain has been removed.
5. If necessary, repeat the process with a fresh piece of filter paper until the stain is completely removed.

Reasoning:
The extra stain is removed by soaking the slide with filter paper because the filter paper has a high absorbency. It can effectively absorb liquids, including stains, due to its porous nature. When the filter paper comes into contact with the extra stain, capillary action takes place, causing the stain to be drawn into the pores of the filter paper. This helps in removing the excess stain from the slide, leaving behind a clean surface for observation.

Soaking with filter paper is preferred over other options mentioned in the question because:
- Running water (option A) may not be sufficient to remove the extra stain completely, and it can also wash away the desired stain.
- Evaporation by heating (option B) may not be suitable as some stains can be heat-sensitive and may lose their color or structure when exposed to high temperatures.
- Drying the stain under sunlight (option D) may not be effective as sunlight alone may not have enough drying power to remove the stain.

In conclusion, soaking the slide with filter paper is the most appropriate method to remove the extra stain, ensuring a clear and accurate microscopic observation.