a means no takebacks are allowed for this choice.
The explanation can be found in the game's code. For example, lines 544 through 548 of
/win/tty/wintty.c (windows version, console menu) state:
/* this prompt string ends in "[ynaq]?":
y - game picks role,&c then asks player to confirm;
n - player manually chooses via menu selections;
a - like 'y', but skips confirmation and starts game;
q - quit
so it appears that choosing the
a option does the same as choosing the
y option (randomize your role), but does not allow you to confirm or cancel if you got a combination you don't want to play, instead it will immediately start the game.
Looking a bit further at line 950:
getconfirmation = (picksomething && pick4u != 'a' && !flags.randomall);
getconfirmation is always false if
pick4u (containing the player's choice) is set to
a. Thus the program will not run the code asking for confirmation, instead leaving the function and presumably going on to setup the game.