Final answer:
Rotational motion is the least common type of motion in membranes, whereas translational and vibrational motions occur more frequently due to the dense packing of molecules which constrains rotational freedom. Option number 2 is correct.
Step-by-step explanation:
The type of motion least common in membranes is rotational motion. In the context of molecular motion within membranes, molecules often undergo translational motion, where they move from one location to another, and vibrational motion, where they move back and forth or oscillate around a fixed point. Rotational motion, which is spin or turning around an axis within its own structure, is less common due to the restricted environment within the membrane where such rotations are constrained by the dense packing of molecules and their interactions with one another.
Translational motion involves a body moving from one place to another without rotating, which is quite common in membranes as it can include lateral diffusion of molecules across the membrane. Linear motion, as described by Essential Knowledge 4.A.1, covers the displacement, velocity, and acceleration of an object's center of mass, and is generally common in various molecular movements in membranes.