Final answer:
Switching between lipid layers (flip-flop) does not occur spontaneously in biological membranes and requires enzymes; in contrast, lateral movement, rotation, and flexing of hydrocarbon chains do occur spontaneously.
Step-by-step explanation:
The student's question asks about the types of movements that occur spontaneously within biological membranes, specifically which movement does not occur spontaneously. The correct answer is (a) switching between lipid layers, also known as flip-flop, which is energetically unfavorable and occurs very rarely without the assistance of specific enzymes called flippases and floppases. In contrast, (b) lateral movement, (c) rotation, and (d) flexing of hydrocarbon chains all occur spontaneously within the phospholipid bilayer of a cell membrane.