You can create questions with polar coordinates by inserting a polar grid as a background image.
For example lets create the following question in which students need to mark a point. We will use graphing type. Plot the point at r=1 and theta of 90 degrees.
First, upload a background image using the Advanced Options menu on the left side of the page (you may need to enable POWER USER on your profile page if these options are not present on the left side.)
Then provide the image URL, and in this case the image was not perfectly centered with the graphing grid as I could see by zooming in. So I experimented to find that an offset of -0.24 was needed.
Then hide the grid and set the snapping distance:
In this case, a grid of 0.1 units works sufficiently for the current question.
Finally, mark the correct answer with point "A" and check it in preview mode.
If you need know details on how to create graphing questions, please see here.