The 2003 Lincoln LS V6 has two ignition coils. They're not individually labeled A, B, C, or D. They are usually located on top of the engine, near the spark plugs they serve. There's one coil pack for cylinders 1, 2, and 3, and another for cylinders 4, 5, and 6. To pinpoint which coil is which, you'll need a repair manual or a diagram specific to that year and engine. These diagrams will show the coil pack placement and which cylinders each one controls.
Without a diagram, trying to determine "coil D" is impossible. If you're having a problem with one of the ignition coils, it's best to consult a repair manual or a qualified mechanic. They can correctly identify the faulty coil and replace it.
