During a run-up, what does it indicate if there is no RPM drop when turning the engine BOTH to RIGHT?

When performing a run-up, observing no RPM drop while switching to the right magneto indicates that there is a potential issue with the ignition system, specifically related to the left P-lead. This lead is responsible for grounding the left magneto. If the left P-lead is not grounding properly, the left magneto will not be able to operate, and thus there will be no drop in RPM when switching to the right magneto. This leads to the conclusion that the right magneto is still functional, but the left magneto is not adequately engaged due to the grounding issue.

In this context, while the other options may suggest different scenarios regarding magneto performance, they do not align with the specific situation where RPM remains unchanged when the engine is switched solely to the right magneto. Understanding this relationship between the P-lead grounding and engine performance during the run-up is vital for diagnosing potential issues with the aircraft's ignition system.