Black ink, in full.
Use a black ballpoint pen – no pencils, no magic markers – and fill in the oval completely. Don’t use blue, red, green or any other color. BLACK. Be sure you’re using an ink pen, as well. Don’t use a pencil, and don’t use a marker.
Finally, completely darken the ovals that correspond with your choices – no X’s, no check marks. Leave no doubt what you’re voting for.