D is the best answer.

The question asks what would strengthen the author’s claim that a new strategy for unionization has emerged since the mid-1970’s. Lines 30-31 cite the appearance of the new strategy. The paragraphs that follow describe the changed circumstances that provided a context for such new strategies. Lines 70-76 explain precisely how these changed circumstances created a reason for new unionizing strategies. The author’s claim would be strengthened if it could be shown not only that there are such new circumstances, but that the old circumstances discussed in the first paragraph have become less important further necessitating the adoption of a new strategy in place of an old strategy suitable to those older circumstances.