Liquefaction occurs when:
a. ground motion causes loose, sandy soil to act like a fluid.
b. rivers are created at fault lines.
c. flooding occurs as a secondary effect of an earthquake.
d. water from a seiche washes away soil.

The correct answer is A. Ground motion causes loose, sandy soil to act like a fluid. 

Liquefaction by definition is a phenomenon where the soil loses strength in response to applied stress, most commonly earthquakes. This phenomenon is best observed in loose, sandy soils because sand when compressed, can act as a strong base for buildings. When that sand is below sea level, during ground movements water can get between the grains of sand and build up. If the build up is big enough to get between the grains of sand it causes them to loose the stability and compactness, and then we see the phenomenon of liquefaction, sometimes even similar to quicksand.