Lets begin with the basics..
We get our IAS (dynamic pressure) from the difference between Total pressure and Static pressure. This has to be corrected because of the position of the pitot tubes and static vents, aircraft configuration, aircraft attitude to the airflow, together with some almost negligible instrument error, to CAS. Next step is correcting CAS for temperature, pressure and density to get TAS. Then from TAS we can calculate our Mach.
At higher airspeeds IAS must be corrected for compressibility in our tubes and vents. This gives us EAS. So EAS=IAS at low airspeeds and EAS<IAS at high airspeeds. With increasing altitude we have to increase our IAS in order to prevent going into a low speed stall. This means that theoretically we can keep a constant EAS while climbing and IAS will increase.. As you already have found a graph including the rest (IAS, TAS and Mach) i wont bother more. You will just have to draw a straight line on the left side of IAS begining from the same point as the other speeds and then spread it upwards.
So with increasing altitude,
EAS<IAS<TAS<Mach
Now for the Best L/D speed. L/D max is obtained at a specific angle of attack which corresponds to a specific IAS for a given weight. That is for level flight. If we increase altitude we have to increase IAS and this ofcourse will increase Drag too. This means that at higher altitude L/D max is found att a higher IAS (supposing weight is being kept constant). But as described above, EAS will always be lower than IAS and therefore the answer they would like you to give them would be EAS is lower than best L/D speed with increasing alt.
In my opinion, this is a wrong question because compressibility is something we get in our instruments, not outside in the free airflow so the relation between L/D and speed is probably more right for EAS than it is for IAS. Secondly, the same graph-drag curve could be plotted were instead of IAS is EAS, giving us the best L/D speed in relation to EAS.The same applies for TAS, etc! A good example for you to understand what i mean is, if we want to measure something, say a wooden stick, we could use m,ft,in,nm,st.m..etc but it still would have the same lenght..
The sum is, EAS, IAS, CAS, Mach are mathematically linked to each other, and L/D max is a just a measure of aerodynamic efficiency.
Jetpipe.
