The major difference between configuration A and B is that busybox is configured for most functionality in configuration B and to minimize memory usage in A. In this case the difference, in terms of memory usage, is 0.7MB. This is the price for including a number of Unix command line tools making your embedded Linux system look and feel more like a Linux OS on your workstation.
These two configurations one would find in standalone embedded systems which typically collect/process data or in server type applications. Configurations C and D includes graphics functionality.
Configuration C has directfb support (console graphics) which enables graphical primitives for the application at a lower level. The introduction of basic graphical functions to the system doesn't add much to the system complexity, merely 1.6MB.
It's when X-windows is included in the system that memory consumption and complexity grows significantly. X-windows definitely provides versatile and generic functionality but results in a cramfs file system size of just above 25MB.
Both configurations C and D are relevant for systems which are used for presentation of data. Building an application on configuration C may require some more effort at development time since directfb doesn't provide as much functionality as X-windows, but may pay off in the long run since it's less complex to maintain.
