Final answer:
At extreme pressure and low temperature, the state of substance X would most likely be a solid. This conclusion aligns with how phase diagrams depict state changes under varying conditions of temperature and pressure.
Step-by-step explanation:
Given that a small sample of pure substance X is held at extreme pressure and low temperature, the state of the substance would most likely be solid. Extreme pressure pushes particles close together, and low temperature reduces the energy of the particles, which both support the formation of a solid state. This is in line with general phase diagrams that depict substance states under various conditions of temperature and pressure. The solid state is typically denser than the liquid state for most substances, and great pressure applied to a liquid would typically force it into a solid form.
The process where a substance changes directly from a gas to a solid without becoming liquid is known as sublimation, and it can occur under certain conditions. However, in the scenario described, as the pressure is increased from a gaseous state, solidification would occur before melting into a liquid happens due to the low temperature maintaining the particles in a close arrangement indicative of a solid.