Which EICAS message only appears on engine indications as required?

The correct answer highlights the EICAS message that specifically pertains to engine indications that may be necessary for monitoring the engine's condition under certain circumstances. In this case, the N1 FAN VIB message is relevant because it focuses on the vibration levels associated with the engine fan. Its occurrence provides critical information to the flight crew, ensuring that they are aware of any potential issues that could affect engine performance or safety.

The other options relate to different aspects of the engine or aircraft monitoring systems. The N1 REV and APR messages, for instance, indicate thrust reverser status and APR (alternate thrust) status, which are broader in scope and appear based on operational conditions rather than strictly on engine indications. The HOT on ITT gauge message provides a temperature warning but does not focus exclusively on engine performance indicators in the same manner as the N1 FAN VIB message. Lastly, the N2 gauge turning WHITE when wing anti-ice is on addresses a functionality of the aircraft’s anti-ice system rather than direct engine performance, making it less pertinent to engine indications as required compared to the targeted application of the N1 FAN VIB alert.

Thus, the specificity and relevance of the N1 FAN VIB message to engine status monitoring make it the correct choice