The correct answer is C) Reduces plate stress. The flush method of welded shell plating, where the plates are joined together without overlapping, does not directly reduce plate stress. The advantages of the flush method are that it reduces weight (option A) and frictional resistance (option B) of the vessel, and it maintains practically 100% of the tensile strength at the joints (option D). However, it does not inherently reduce the plate stress itself, which is why option C is the correct answer. The other options are correct because the flush method eliminates the need for overlapping plates, which results in lower weight, reduced frictional resistance, and preservation of the tensile strength of the welded joints.