In this sentence, the correct word is "across." The phrase "across the page" makes sense because it indicates the line ran from one side of the page to the other, dividing it into two. The other options don't fit contextually:
- "of" would not work because it doesn't convey the idea of something extending over a surface.
- "between" would imply a separation between two distinct things, which doesn't work with a single line on the page.
- "about" would imply approximation or location, which is not suitable for describing a line running through the page.
Thus, the correct answer is (A): across.