|A is usually the floppy drive..you cannot defrag it.|
D is usually the recovery partition (don't defrag it) or an optical drive (cannot defrag it).
If D is a normal OS or storage drive in your system, then definitely defrag it regularly for optimum performance. For automatic defragmentation, there are third party options that are of the 'set once and forget' type which make life much easier. These utilities also typically defrag more comprehensively than the native Windows defrag utility.