How Was Mercury's Unusual Spin-Orbit Coupling Discovered

Multiple Choice

How was Mercury's unusual spin-orbit coupling discovered?

A) Earth-based telescopes watched the movement of surface features on Mercury as it rotated.
B) It was determined by watching the phases of Mercury as observed from Earth.
C) It was not observed until a spacecraft was put into a permanent orbit around Mercury.
D) Radar waves were bounced off Mercury from an Earth-based radio telescope, and the Doppler shift of the reflections was measured.