What does a yellow arc indicate on an aircraft instrument?

A yellow arc on an aircraft instrument is used to indicate a caution or limited flight operations range. This visual guide serves to alert pilots that the parameters displayed fall within a range that is not optimal for safe operations and may entail increased risk. Essentially, while it may not prohibit operation within this range, it does suggest careful consideration and monitoring of aircraft performance if flying in that zone.

In contrast, the other options represent different meanings in a flight operation context. An optimal operating range for efficiency is typically indicated by a green arc, while the normal operations range would also fall under the green area on the dial. A warning for critical failure conditions would generally be denoted by red markings or indicators, clearly indicating danger or the need for immediate corrective action.