When assigned a speed of 210 KIAS while being vectored for an approach, when is it permissible to change that speed?

The correct answer is the moment at which a new speed is communicated to you or when you receive an approach clearance. The reason this is the appropriate time to change your assigned speed is due to the structured nature of air traffic control instructions. Controllers assign specific speeds to manage traffic flow and ensure safety.

Until a new speed is explicitly provided or you are cleared for the approach, maintaining the instructed speed is crucial for staying coordinated with the sequencing of other aircraft. This helps prevent misunderstandings or conflicts that could arise from uncoordinated altitude or speed changes.

Options relating to changing speed at the initial approach fix or at any time during the approach lack the necessary clarity from air traffic control, and changing speeds after passing 10,000 feet does not align with the established rules for altitude and speed management, which are based on current ATC instructions rather than arbitrary altitude thresholds.