Which of the following describes structures that are unable to move from their position?

The term that best describes structures that are unable to move from their position is "sessile." In biological contexts, sessile organisms or structures are fixed in one place and do not have the capability to move actively. Common examples of sessile organisms include barnacles, certain types of corals, and many plants that are anchored to the substrate.

The term implies a specific relationship to mobility; sessile organisms typically rely on water currents or other external forces for nutrient uptake and reproduction rather than self-propulsion. This adaptation can be advantageous in stable environments where mobility is not necessary for survival.

In contrast, the other terms do not accurately describe immobility. "Mobile" refers to organisms that can move freely and actively. "Static" might imply being stationary, but it doesn't capture the biological context properly. "Nomadic" describes organisms that are always on the move, typically in search of food or suitable habitats, which is the opposite of being sessile.