Final answer:
c) Substance with changing viscosity
A thixotropic gel is a substance with changing viscosity, behaving like a solid under static conditions but like a liquid when stirred or shaken.
Step-by-step explanation:
A thixotropic gel is a substance with changing viscosity. These are fascinating materials that behave more like a solid under static conditions but become more liquid when agitated or stressed. This behavior is due to the structure of the gel on the microscopic level breaking down under stress, making it easier to flow. Such materials are sometimes encountered in manufacturing processes and everyday products like ketchup, which stays in place when still but flows easily when the bottle is squeezed or shaken.
Addressing the student's question more specifically, the answer is c) Substance with changing viscosity.