The verification question on Voxelmanip Classic was introduced in order to combat griefing. When first joining the server, the welcome dialog is shown, and the verification form can be opened by clicking the formspec hypertext link at the bottom.
It is a relatively simple maths question, yet ever since the verification question was introduced the rate of griefing has significantly decreased to almost zero. This is most likely due to most griefers being children that don't grasp basic maths, and cannot contribute in any meaningful manner to the server other than holding down the mine button.
Currently, the question is the answer to this problem:
sqrt(floor(pi) * 48 + ( sin(pi) * e ))
It may look more advanced than it actually is. Remember the exact trigonometrical values and try to simplify down it to a simple square root.