Id say B since with a loop command block it can lock everyone out of a server.
But as somebody who uses command blocks semi-frequently, I'd say C because with run it lets you use literally every other command.
Yeah, I was actually working on a multiplayer minigame server just a few days ago. I had to learn how to use /execute through a few tutorials since they removed the 'testfor' command.
True, /execute is the most versatile command in the game, but I still kinda wish we got /testfor command, sure nobody uses it. Until I needed to use it for one simple thing lol
You hear it got removed in the 1.13 update?