A(n) is an instrument used to detect and measure earthquakes.
a. seismograph
c. gps
b. volcanometer
d. anemometer

Im pretty sure its a seismograph i think thats it

Seismograph. When the earth shakes, so does the detector in the seismograph. That wiggles a pen which causes a wavey line as the shocks pass through the detector. 
GPS is global POSITIONING SYSTEM - maybe when there's a quake it might be picked up by a gps as a small change in the positions of things.
anemometers measure fluid flow, such as air wind speeds etc.
a volcanometer measures volcanoes