Answer :

On the eve of the Great Depression, the percentage of Americans who had a stake in the stock market was 7.5%.

To arrive at this answer:

1. We identify the given percentages:
- 2.5%
- 57.5%
- 7.5%
- 25.5%

2. Analyze the context: The stock market participation on the eve of the Great Depression, which was in the late 1920s, was relatively low.

3. The correct answer among the options provided is 7.5%, as this figure accurately reflects the portion of Americans involved in the stock market during that period.

Therefore, the percentage of Americans who had a stake in the stock market on the eve of the Great Depression was 7.5%.