The correct answer is B) in the bottom return mode. A Doppler speed log measures the vessel's speed over ground by analyzing the Doppler shift of the acoustic signals reflected off the seafloor. This bottom return mode provides an accurate speed over ground reading, as it is not affected by currents. The other options are incorrect: A) is incorrect because the Doppler speed log does not indicate speed over ground at all times, only in the bottom return mode. C) is incorrect because the Doppler speed log can provide speed over ground readings even when there is a current present. D) is incorrect because the volume reverberation mode is not used to measure speed over ground.