Which of the following shapes indicates "no passing"?

The shape that indicates "no passing" is a pennant. This shape is unique in that it is a side-flag or triangular shape that is specifically designed to catch the driver's attention. In the context of traffic signs, the pennant shape typically signifies that it is illegal to pass other vehicles in that area. This is often seen in winding roads or areas where visibility is limited, helping ensure the safety of both the driver and other road users.

Regarding the other shapes, the pentagon is commonly associated with school zone signs; the inverted triangle is used for yield signs, indicating that drivers must give way to other vehicles or pedestrians; and rectangles generally indicate regulatory signs like speed limits or information signs. Each shape has its specific meaning, but for the indication of "no passing," the pennant shape is the correct one.