Wrong Way sign
You are travelling against the flow of traffic. Stop, then back out or turn around when it is safe.
How to recognise it
This sign is a wide rectangle in red, the colour reserved for prohibition and stopping. These carry the force of law. Ignore one and you have broken a traffic rule, not just a guideline. Most are white rectangles with black text, and the handful that are not, the stop and yield signs, get their own unmistakable shapes.
What the test wants you to know
Wrong Way is the second line of defence after Do Not Enter. Seeing it means you are already on the ramp or road facing oncoming traffic, so the correct answer is always to stop and get out, never to continue looking for a turnaround.
The sign people confuse it with
Wrong Way gets mixed up with Do Not Enter more than any other pairing in this category. Put them side by side and the difference is obvious, which is exactly why it is worth doing once rather than meeting them cold in the exam.
Practise it
Reading about a sign is not the same as recognising it in half a second with an examiner watching. Every state road signs test on this site shows the sign and asks you to name it, then shows four faces and asks you to pick the right one.