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.