The correct answer is B) Short splice. For repairing a parted synthetic fiber mooring line, the short splice is the best option. The short splice is a quick and effective way to join two ropes together without significantly increasing the diameter at the splice. This is important for maintaining the integrity and strength of the mooring line. In contrast, the other splice options like the long splice or Liverpool splice would create a thicker, bulkier section that could interfere with the mooring operation. The short splice is the preferred method for repairing synthetic fiber lines as per industry standards and best practices for mooring operations.