This is the other half of the paired questions 2 and 3. Here, you were asked to give a short list of possible driving forces, not energy sources, for plate tectonics. Your textbook discusses five possible driving forces for plate tectonics:
A good way of visualizing this idea is to imagine holding a garden hose under a window screen, with the open end of the hose pointing at the screen from below. Turn on the water and watch what happens. Your hose is the equivalent of a thermal plume, and the screen takes the place of a plate. Some of the water will make it through the holes in the screen -- this water is the equivalent of the magma at hot spots. Most of the water, however, will flow along the bottom of the screen until it gets to the point where gravity makes it pull away from the screen and get your feet wet. That water is the equivalent of the ``return flow'' from the top of the mantle in the Thermal Plume model.
A number of people said that the driving force was seafloor spreading. While this may appear identical to the ``ridge push'' model above, it is in fact different. Seafloor spreading is a model which explains the existence of the magnetic stripes discussed in Question 1 (among other things) -- it is an integral part of plate tectonics. However, seafloor spreading only happens because something is forcing the plates to spread apart, either by pushing them apart, pulling them apart, or whatever. It is that force, whatever it may be, which causes seafloor spreading. Answering the question with ``seafloor spreading'' got you 10 points.
I thought you might find it interesting to hear what geoscientists think about this problem. I think most geoscientists would agree that the driving forces for plate tectonics are not completely understood. Just 10-15 years ago, the most accepted model was that the magma rising to the mid-ocean ridges pushed the plates apart, and that subducting plates had nothing to do with the picture (the ``ridge push'' model). Today, I think it's safe to say that most geoscientists think that the most important driving force is actually not ``ridge push'', but ``slab pull'' -- the weight of the cold subducting plate pulling all the rest of the plate behind it.
My own personal suspicion is that we will find out that it is a combination of ``ridge push'', ``slab pull'', and convection dragging the plates. You'd have to ask Lisa what she thinks.
Your book discusses this problem in Chapters 19 and 20, and you can look at figure 20.27 particularly.