The offense of Ananias and Sapphira was that they lied to the Holy Spirit by keeping back some of the proceeds from the sale of their land, while pretending to give the full amount to the church. They had sold the land and agreed to give all the proceeds to the church, but they secretly kept some for themselves. When Peter asked them about it, they lied and claimed that they had given the full amount. Peter then rebuked them for lying to the Holy Spirit and they both died immediately. Therefore, the correct option is "they kept some of the proceeds of the sold land."