Sink B. Sink A is too close to the oft used urinals, thus increasing the chance of civilian foot traffic interfering with your mission. Objective C is too far to hump a toothbrush AND toothpaste to. You ain't no Army Ranger or Navy SEAL! Sink B is the best compromise between obj/staging point distance, and target isolation. Now spend your time rehearsing insertion (open that door smoothly! Watch those corners!), infiltration/approach (observe the target, check for hostile use of sink!), action at obj (get that toothpaste on that brush before you even arrive, no more than a minute onsite!), and exfiltration (check for incoming on that bathroom door, you don't want those clean fresh chompers inadvertently whacked because you took your eyes off the prize!).
Good luck out there. We expect after-action reports on brush performance/accuracy, and your opinions of toothpaste takedown power (did the plaque go down and stay down?)...
I can't believe I wrote that...