Conciseness isn't the issue here; usage is. The phrase besides that can't be used as an introductory clause. There's some good information about this written at this blog post:
Besides that” means “in addition to that”. You use it to refer to something after having mentioned it before. You can also use it to connect two sentences.
Had choice B been "Although" instead of "Besides that", then conciseness might have come into play. But this sentence:
Besides that you are missing three pages of work, your portfolio appears to be of extremely high caliber.
doesn't make any sense. The only way I can see starting a sentence like that with "Besides that" would be to add a comma, and, more importantly, change the last part to express something negative, to go along with the first part:
Besides that, you are missing three pages of work, and your portfolio appears to be of low caliber.
A sentence like that could be the conclusion of a longer missive:
There's a reason you only got a D in this class: your attendance was very spotty. Besides that, you are missing three pages of work, and your portfolio appears to be of low caliber.