The answer is: A) epicenter.
Seismometer is an instrument that measures ground motions and the point where an earthquake originates (epicenter).
It is build of a mass attached to a fixed base; during an earthquake, the base moves and the mass does not.
Seismograph records the times when the first (P waves) and second (S waves) seismic waves arrive. Then, scientists determine how fast the waves are traveling and at the end calculate the distance from the epicenter to each seismograph.