The correct answer is: C. adjustment of horizontal axis.
The spire test is used to adjust the horizontal axis of a theodolite. The test is performed by sighting the theodolite on a distant object, such as a church spire, and then adjusting the horizontal axis until the image of the spire is centered in the field of view.
The other options are incorrect because they do not require the use of the spire test. The plate levels are adjusted by sighting the theodolite on a level surface and then adjusting the plate levels until the bubble is centered in the vial. The line of sight is adjusted by sighting the theodolite on a distant object and then adjusting the line of sight until the image of the object is sharp. The altitude bubble and vertical index frame are adjusted by sighting the theodolite on a vertical surface and then adjusting the altitude bubble and vertical index frame until the bubble is centered and the index line is parallel to the vertical surface.