Final answer:
The EICAS would likely show 'Three amber dashes' to indicate an invalid signal from a 10th-stage pressure transducer. This convention alerts the crew to unreliable data in a standardized manner without using critical warning or informational message colors.
Step-by-step explanation:
When the EICAS (Engine Indicating and Crew Alerting System) indicates that a 10th-stage pressure transducer signal is invalid, the usual display alteration would likely be a) 'Three amber dashes' in place of the normal transducer reading. The 10th-stage transducer, which is part of the aircraft's pneumatic system, measures the pressure of the air compressed by the engine's 10th-stage compressor. If the signal from this transducer is lost or erroneous, the EICAS would typically show a fault indication, such as amber dashes, to alert the flight crew that the data is not reliable. It is not the convention to display a red warning or blue advisory message for this type of fault, as these colors are reserved for more critical or informational alerts, respectively. And the absence of an indication (d) is generally not a design feature of failure-mode annunciators in aircraft systems.