There is one coil on each cylinder of this motor. Since Ford numbers their cylinders from front to rear, and starts on the passenger side of the motor; coil "D" would be the coil on the # 4 cylinder; which is the one closest to the firewall on the passenger side.
Here's a link to a photo of the coil: (http://www.rockauto.com/catalog/moreinf
o...) It is a piece of cake to replace the coil; just disconnect the wiring harness plug and remove the mounting bolt. Then lift the coil off the spark plug. Installation is the reverse order of removal.
It is recommended to install a new spark plug when a coil is replaced; as the misfiring which typically is created by a defective coil can compromise the performance of a plug; and it may be difficult or impossible to adequately clean a plug which has become fouled by that means. Be sure to use the same brand and part number of plug as the ones now in the motor; unless it is the wrong part for that motor; or you replace all the plugs.