B: It sounds like a reasonable choice. However, if it was possible to make dorfs follow "hold this position" order, I'd suggest trying to lure them to a chokepoint and going with C. As I personally discovered they just lack the discipline to wait and stay in a tactically advantageous position, I'll stick to B.
I was still dabbling with the game and forgot to make multiple successive burrows for civilians to retreat. It all began with a Forgotten Beast that launches freezing breath and suffocates Dorfs, then an Ambush that a human guard fortunately helped to stop. Then, at first siege, after 4 years of peace because of being in a far away glacier, around 20 goblins attacked. I still didn't get the hang of burrows, Magma was just 80 z-levels below the already built residences and thus metalsmithing remained inactive and most weapons were bought or seized from dead goblins all this time.
Then, just a few days after the first siege was defeated, with heavy casualties reducing population from 180, already reduced because of ambush and FB to around 150, antoher FB caused even more rape, and as if it wasn't enough, right after this second FB came a second siege in the form of a full rape squad, with 40 goblins and 6 trolls.
My initial plan was to use a retractable bridge as a chokepoint, and I was confident enough in it to hope the fortress would withstand but the dumb dwarves, in spite of having defined two burrows(one around the archery spots above the walled first entrance and windmill farm and the other just on the tip of the bridge as a chokepoint, while also covered to the sides by walls to secure them from bolts) forbid them from getting down from the bridge and expose themselves. Of course, instead of holding their position, they just did the dumbest thing ever and charged against the horde.
The east retractable bridge was quickly seized, and the trolls smashed the way open to the underground....
After all, due to my foolishness of not having created multiple layers of burrows for civilians to retreat deeper and deeper, and also due to the dumb dwarves not holding their position in the bridge and behind the fortifications and charging instead, getting all of the first line of defenses slaughtered, once this second siege was over, fortunately with most of the perpetrators exterminated, including four of the trolls, only 100 dorfs survived, of which a dozen became cripples for life, although it is impressive what an one-legged elite swordman with a sword in left hand and both a crutch and a shield in right can do.