What squawk code should be used by non-lead aircraft in a nonstandard formation if ATC requires it?

In the context of aircraft operating in a nonstandard formation, the appropriate squawk code for non-lead aircraft when instructed by ATC would typically be to utilize assigned codes. This allows ATC to maintain awareness of each aircraft's position and contributes to safe separation and management of traffic.

Assigning a specific transponder code to each non-lead aircraft helps ATC to identify and track these aircraft individually, despite their formation flying. It helps avoid confusion and enhances situational awareness.

Using standby mode or turning the transponder off would not allow the aircraft to be monitored by ATC, which is critical in busy airspace or during times when precise tracking is required. Unique identifiers do not specifically pertain to the standard transponder codes used in this context, and therefore wouldn't fulfill the need for a precise tracking system.

Thus, the assigned code is the right answer as it aligns with ATC protocols for ensuring safety and coordination amongst aircraft in nonstandard formations.