All of your car's parts have 4 slots which are either empty or include 1-4 of A, B, C or D.
To unlock this, in the Inventory you need to have all parts under 1 category (Ignition, Cooling, Drivetrain or Compression) have a matching letter. Once you do this the achievement will pop, and the letter will go green (and you'll get a bonus % to your performance)
To get new parts you can use the Junkyard, Part Challenges or Store.
Note: The Air System and Electrical categories do not apply as there's only one part under them (and they don't have boxes where the set letter appears, and can't be filtered by Part Set in the Store).
e.g. under Ignition, have something equipped with the letter 'A' for the Spark Plug, Distributor, Nitro NTE, Alternator and Fuel Pump.