Final answer:
Acidic dyes are used for negative staining, as they are repelled by the negatively charged cell walls, creating an outline of the cells.
Step-by-step explanation:
The type of dye used for negative staining is an acidic dye. Negative stains work by repelling away from the negatively charged cell walls, causing a silhouette or outline of the cells against a stained background. Since cells typically have negatively charged cell walls, the negatively charged chromophores in acidic dyes, such as acid fuchsin, eosin, and rose bengal, are repulsed by the cell wall.
This is in contrast to basic dyes, like methylene blue and crystal violet, which are attracted to the cell walls, causing them to be used as positive stains. The answer to the student's question is therefore (a) Acidic dye.