What is the epicenter of an earthquake and how many seismograph stations are needed to determine its location?

The piont of initial perturbation. We need at lest three seismographic stations

The epicenter is the point on the earth's surface vertically above the hypocenter. The number of seismograph stations that are needed to determine its location is three. On each of the seismograph stations, there is S - P which you have to measure in seconds. Hope this helps! 

Ithink the answer is d.