Sentence Correction

OA is D, but in (D) can "it" not refer to the "chain" or the "link" and hence an ambiguous pronoun reference.

Although one link in the chain was demonstrated to be weak, but not sufficiently so to require the recall of the automobile.
(A) demonstrated to be weak, but not sufficiently so to require
(B) demonstrated as weak, but it was not sufficiently so that it required
(C) demonstrably weak, but not sufficiently so to require
(D) demonstrably weak, it was not so weak as to require
(E) demonstrably weak, it was not weak enough that it required

Subject of the main clause (IT) always refers to subject of the subordinate clause (ONE LINK). Hence no ambiguity.

HTH