The correct answer is C) changing the vertical location of the pilot valve bushing. The synchronizing device, labeled as figure "D" in the illustration, is responsible for making fine adjustments to the steam turbine speed. By changing the vertical location of the pilot valve bushing, the synchronizing device can effectively adjust the steam flow to the turbine, allowing for precise speed control. The other options are incorrect because they do not directly impact the steam flow to the turbine. Varying the pivot rod stroke length (A), increasing/decreasing spring pressure (B), or raising/lowering the nozzle block lifting beam (D) are not the primary mechanisms used by the synchronizing device to make the necessary fine adjustments to the turbine speed.