B for me. Pretty easy one, if one's fundamentals are right.
Here is the rule. I just saw it a few days ago.
X is one of Y ( here Y is always a plural noun) + PLURAL verb
ACE are out.
Between B and D, D uses an inordinate amount of "they"'s. Specially the first "they" is not at all required.
B. Icelandic is one of several Germanic languages that are compact when [they are ]written,
Lovely explanation. I also solved it this way but could have never thought of this good a way to explain it.