The correct option is
B why
The sentence indicates that the party is the subject's 'reason' for writing the letter. Thus, the answer must indicate 'reason'. Also, a relative clause is being introduced after the blank, thus, the answer must be a conjunction that introduces the relative clause.
Option A is incorrect. 'because' is a conjunction used to indicate reason but, in this case, it does not fit very well with the word 'reason'. It does not introduce the relative clause in a proper manner. Thus, it is incorrect.
Option B is incorrect. 'for' is a conjunction that is used to indicate 'purpose' and not 'reason'. For example, running for exercise.
Option C is correct. 'why' is a conjunction that is used to indicate reason. Also, it is normally used after reason to introduce a relative clause. Thus, it is a better fit that all the other options and is the correct answer.
Option D is incorrect. 'of' is a conjunction that is used to indicate 'possession or association'and not 'reason'. For example, property of the church; king of France.