The correct answer is C) in the volume reverberation mode. A Doppler speed log measures the speed of the vessel through the water by using the Doppler effect, which measures the change in frequency of the sound waves reflected off particles in the water. The volume reverberation mode is the standard operating mode for a Doppler speed log, as it can accurately measure the speed through the water regardless of the presence or direction of current. The other options are incorrect because: A) a Doppler speed log does not indicate speed through the water at all times, as it requires the presence of particles in the water to measure the Doppler shift; B) the Doppler speed log is not limited to situations with no current; and D) the bottom return mode is used for different purposes, such as measuring depth, and not for determining speed through the water.