What type of marking indicates that entry is completely prohibited?

The marking that indicates entry is completely prohibited is a red circle, which is widely recognized as a universal symbol for restrictions or prohibitions. This visual cue signifies that access to an area is not allowed, ensuring that individuals are aware that they should not enter under any circumstances. The use of a red circle effectively highlights the seriousness of the restriction, making it easy for individuals to understand that disregarding this marker could lead to legal consequences or safety risks.

The other options do not carry the same level of authority regarding prohibition. For instance, a yellow circle might indicate caution or a warning but does not necessarily mean that entry is forbidden. Similarly, a green square could denote permission or safety, and a blue rectangle typically signifies information rather than restriction. Thus, the red circle is the clear and appropriate symbol to indicate complete prohibition of entry in a designated area.