Absolute twaddle but it is in the question bank and the examiner insists that answer “Saint-Venant, taking into account the air compressibility” is correct.   Firstly an anemometer measures the speed of the air (wind speed) and not the air speed of the aircraft.  Secondly the Saint-Venant formulae deal with pressure in flood water and pressures exerted on dams; the air speed indicator is calibrated assuming standard atmosphere conditions using Bernoulli’s formula.