Final answer:
The property of the upper mantle that allows for the movement of tectonic plates is its ability to undergo slow convection, which is not explicitly described by any of the provided options in the question. Mantle rock can flow slowly due to the semi-plastic behavior under high temperature and pressure. Option number 3 is correct.
Step-by-step explanation:
The substance in the upper mantle that allows tectonic plates to move is not mentioned explicitly as gravel, sandy clay, liquid state, or pockets of water. However, based on the provided information, it's the slow convection of the mantle that enables this movement. During convection, heat is transported from warmer material rising up and cooler material sinking, which facilitates the motion of the tectonic plates, reshaping continents over geological time frames.
Mantle rock can deform and flow slowly, but it is not in a liquid state. Hence, the correct option that explains the property of the upper mantle that allows plates to move is not provided in the choices A to D of the question. A deeper understanding of the mantle reveals that it is solid, but due to extreme temperatures and pressures, it can behave plastically and enable the motion of tectonic plates.