Explanation:

- Pratima's Typing Speed: Pratima completes work in 10 hours, so she does \( \frac{1}{10} \) of the work per hour.

- Diksha's Typing Speed: Diksha completes work in 15 hours, so she does \( \frac{1}{15} \) of the work per hour.

- Pratima's Contribution: Pratima works alone for 4 hours, completing \( 4 \times \frac{1}{10} = \frac{4}{10} = \frac{2}{5} \) of the work.

- Remaining Work: \( 1 - \frac{2}{5} = \frac{3}{5} \) of the work is left for Diksha.

- Time for Diksha to Complete: It takes Diksha \( \frac{3/5}{1/15} = 9 \) hours to finish the remaining work.

- Option Analysis:

- Option 1, 8 hours: Too short.

- Option 2, 10 hours: Longer than needed.

- Option 3, 9 hours: Correct duration for remaining work completion.

- Option 4, 7 hours: Insufficient time.