She was very grateful ___ her friend for all the help with the arrangement of the party.a) forb) withc) ofd) to

To determine the correct answer to fill in the blank in the sentence, we need to consider the common prepositions used in expressions of gratitude.

Problem Breakdown

1. Identify the structure of the sentence.
2. Analyze which preposition fits best in the context of expressing gratitude.

Analysis

1. "for" - This preposition typically indicates the reason or grounds for gratitude (e.g., "thank you for your help").
2. "with" - This preposition would suggest collaboration rather than gratitude and does not fit well in this context.
3. "of" - This preposition is not commonly used with expressions of gratitude.
4. "to" - While "thank you to" can be used, it is generally more appropriate to specify the reason with "for."

Conclusion

The most suitable preposition to convey the speaker's gratitude towards her friend for the help provided is "for."

Final Answer

The correct answer is a) for.