What does the "white arc" on an airspeed indicator signify?

The "white arc" on an airspeed indicator signifies the flap operating area. This area shows the range of speeds at which the pilot can safely extend and operate the flaps. It indicates the lower limit, or the stall speed with flaps extended, and the upper limit, or the maximum speed at which flaps can be safely deployed. Understanding this is crucial for ensuring safe and effective airplane operation during situations that require flap extension, such as during approach and landing.

In contrast, the other options refer to different speed ranges but do not accurately describe the function of the white arc. Normal flight operating speeds typically fall within a broader range that includes both flap and clean configurations. Emergency descent speeds involve specific operational situations not indicated by this arc, which is focused solely on flap use. Lastly, while optimal cruising speeds for fuel efficiency are important for flight planning, they are not represented on the airspeed indicator in this manner.