If EAS is RAS(CAS) corrected for compressibility, then at higher altitudes compressibility will increase (due to the higher TAS at higher altitudes).
So if compressibility increases, the RAS(CAS) will over-read when compared to the EAS.
Therefore EAS will be less than the RAS(CAS).
Therefore answer (b) is correct.
I'm assuming that 'X' is an ASIR/IAS, which for the purpose of this discussion can be assumed to be equal to RAS(CAS).
Here's a repeat of my answer to a previous post in a different forum:
This is my understanding of the various different speeds that can be used and their corrections:
ASIR - Air Speed Indicator Reading; the uncorrected reading of the instrument.
IE - Instrument Error; a correction made for errors in the construction and calibration of the instrument.
IAS - Indicated airspeed; ASIR corrected for IE.
PEC - Pressure Error Correction; a correction made for erroneous static port measurements caused by the position of the port and the local airflow disturbance cuased by airspeed, altitude and configuration.
RAS - Rectified Air Speed; IAS corrected for PEC.
CAS - Calibrated Air Speed; US term for RAS.
CE - Compressibility Error; the error induced by the increase in apparent air density at high speeds due to the compressibility of air
EAS - Equivalent Air Speed; the RAS or CAS corrected for CE. This is the speed that the aircraft 'thinks' it is flying at.
DE - Density Error; an error due to the fact that pitot pressure at a given speed varies with air density. The instrument is calibrated to read correctly with an asssumed air density of 1.225kg per cubic metre (i.e. the air density at sea level in the ISA)
TAS - True Air Speed; EAS corrected for DE.
So, the full sequence is:
ASIR
IE
IAS
PEC
RAS(CAS)
CE
EAS
DE
TAS
Hope this helps!
Eck