The correct answer is (a) its viscosity decrease.
Viscosity is the defined as the state of being thick. The viscosity of lava is very important to determine its behavior.The fluid has the property to become thin as the temperature increases. The thin nature of liquid means decreased viscosity. Greater temperatures in lava tend to destroy polymerized bonds within the lava, which promotes more fluid behavior. Hence, the increase in temperature decreases its viscosity.
