D is the best answer.

In the first paragraph of the passage, the author asserts that efforts to unionize public-sector clerical workers prior to 1975 were limited and then goes on to describe these limited efforts. In the second paragraph, the author asserts that a new strategy developed after 1975 and cites an increase in union membership among public-sector clerical workers. The author begins the last paragraph by asking what can explain this increase in union membership, and then proceeds to provide an explanation. Thus, the passage is primarily concerned with analyzing and explaining the increase in unionization among public-sector clerical workers.