The correct answer is A) product during the final phase of loading. Line displacement is a procedure performed at oil terminal facilities when there is a change in the product being loaded. This is done to ensure that the lines are properly flushed and cleared of the previous product before the new product is loaded. This is typically done during the final phase of loading to transition between different products without contaminating the new product. The other options are incorrect because line displacement is not performed before the start of loading (B), when changing cargo lines (C), or after the final discharge (D). These scenarios would require different procedures to ensure product quality and safety.