Which of the following indicates rapidly changing wind direction in a weather report?

The indication of rapidly changing wind direction in a weather report is represented by "VRB," which stands for variable winds. When winds are described as variable, it means the wind direction is not consistent and can shift quickly between different compass points. This is particularly significant because variable winds can affect flight operations, especially during takeoff and landing when pilots need to be aware of changing conditions.

The other options provide information related to wind but do not indicate rapid changes in direction. For example, "SQUALLS" refer to sudden increases in wind speed, typically associated with thunderstorms, but do not specifically indicate changes in direction. "GUSTY" describes winds that are characterized by sudden increases in speed but do not imply variability in direction. "CALM" indicates no significant wind, which obviously would not reflect changing wind conditions at all. Therefore, "VRB" is the correct choice to denote rapidly changing wind direction.