To arrange the sentences in a logical order, we need to consider the flow of the letter. It's best to start with a thank-you, then express gratitude, acknowledge challenges, mention future plans, and finally, end with a closing remark.

Therefore, the correct order is: b-a-c-d-e

Here's the letter with the sentences rearranged:

Hi Daniel,

b. Thanks a lot for the tips you gave me during our last study session. a. You really helped me focus on improving my writing skills. c. It was challenging, but I feel much more confident now. d. I still need some practice, though, especially with grammar. e. Hope we can meet up again soon for another session!

Best, Laura