Explanation:

- You're buying eight copies of a book on credit to be paid after one year. You can also buy ten copies right now with cash for the same amount.

- This implies there's extra cost (interest) when buying on credit because you get fewer copies.

- Let's denote the total cash price for 10 copies as $S$. Since 8 copies are equated to the same amount after adding interest, the price per copy remains constant.

- After one year, the cost for 8 copies amounts to $S$, so the price for 8 copies $=8\times \text{price per copy}=\frac{8}{10}S$.

- The difference between the year-end payment and cash payment is taken as interest.

- Interest is $S-\frac{8}{10}S=\frac{2}{10}S=\frac{1}{5}S$.

- The rate percentage of the interest can be calculated as $\left(\frac{1/5}{8/10}\right)\times 100=25\mathrm{\%}$.

- Therefore, Option 2 is the correct choice: 25%.