Answer:
Dr. Jekyll wrote the letter that he claims was written by Mr. Hyde.
Step-by-step explanation:
Based on the information provided in this passage, it seems likely that Dr. Jekyll wrote the letter that was allegedly written by Mr. Hyde. This is supported by the information that Mr. Guest gives to Mr. Utterson. He says that the letters are unusually similar, and in fact, are identical in many points. This shows that it is likely that the letters were written by the same hand.