Refer to figure.
EASA learning objective 022.02.06.01.03 states: ‘Explain the operating principles of an ASI (as appropriate to aeroplanes or helicopters).’

Pilots are interested in an airspeed relating to the aerodynamic performance of an aircraft, which depends on the flow rate of air around the wings. To achieve this an airspeed indicator must measure dynamic pressure.

An airspeed indicator is a basic instrument which can measure dynamic pressure and displays indicated airspeed (IAS). Dynamic pressure cannot be measured directly as it's a combination of total (pitot) pressure and static pressure. An airspeed indicator is fed total pressure, from a forward facing pitot tube, via a pipe into a sealed capsule in the instrument. Static pressure from static ports is fed to the instrument case. As an aircraft increases and decreases airspeed the total pressure capsule will expand and contract. Because the pressure capsule is surrounded by static pressure the movement of the capsule is only due to changes in dynamic pressure. The capsule movement is translated by a suitable system of linkages and levers to move a pointer over an airspeed scale.

Obviously a mechanical system such as this will be prone to instrument errors as well as any position errors in the pitot static system. Indicated airspeed when corrected for these errors is called calibrated airspeed (CAS). Aircraft fitted with air data computers can apply instrument and position error corrections to output CAS.

TAS → INCORRECT. This is the actual speed of the aircraft through the air and is IAS corrected for instrument, position, compressibility and density errors.

EAS → INCORRECT. Equivalent airspeed is CAS corrected for compressibility error

CAS → INCORRECT. An ASI displays indicated airspeed (IAS), CAS is IAS corrected for instrument and position errors. EFIS airspeed indications supplied by an ADC are CAS but are often still referred to as IAS.

IAS → CORRECT. The clue is in the name of the instrument, an airspeed indicator displays indicated airspeed.