When looking at the dictionary, you find that intersperse means to sprinkle, or expand something between things, or among things. It also means to place at intervals or spread at intervals, or in between something. When looking at other words that express the same idea as this one, you find that closely related words are interweave, which means interlace something. Interspersing can then mean connecting things through the use of something in between, for example, using pictures, or drawings, to connect and emphazise an idea. Interspersing, in this example, would be placing pictures throughout, or among, text, to help emphazise a point, or clarify a point. This is why the correct answer is B, to insert at intervals.
