Answer:
The best revision for the following sentence is:
B) We have identified a problem with our expense sheet, but we will solve it.
Explanation:
In e-mail, letters, messages, memos, and other business documents intended for a busy audience who appreciates getting the information they need quickly and easily, replacing a phrases with a word in a formal tone, detracting from adding meaning: "we have identified" instead of: "I am sending" or "we need to inform you", keeping the sentence just as it is with certain rhythm and concise.
Otherwise in option 1. I am sending you this letter to inform you that we have experienced an unexpected surprise within our expense sheet, but we are positively certain we will solve it.
The sentence has a redundant pair: "send" and "inform", it is “wordy,” and includes irrelevant “fillers” as "unexpected" that makes it lack of conciseness in the substantive information, not getting to the point promptly. I also includes passive voice which is not a grammatical error, but it often leads to using more words than necessary frustrating or confusing the audience; and it has prepositional phrases overuse: "that", within", "but" making this sentence clunky and unclear.
In option A) We need to inform you that we have experienced an unexpected surprise with our expense sheet, but we are midway to an end result.
The sentence has unnecessary modifiers: "unexpected" and "end", and passive voice again.
In option C) I am sending you this letter to inform you that we have a problem with our expense sheet, but we are positively certain we have a solution.
The sentence has unnecessary modifier: "positively".