Final answer:
The movement known as flip-flop requires enzymatic activation and the expenditure of ATP because it involves phospholipids moving across the hydrophobic core of the membrane, which is energetically unfavorable.
Step-by-step explanation:
The movement of membrane phospholipids that require enzymatic activation and ATP is A) Flip-flop. This refers to the process by which phospholipids move from one leaflet of the bilayer to the opposite leaflet. This movement is energetically unfavorable because it involves moving the polar head group through the hydrophobic core of the membrane, and thus it requires an enzyme, known as flippase, and the expenditure of ATP to occur. Unlike flip-flop, B) Lateral diffusion is the rapid movement of phospholipids within the same leaflet, and it occurs without the need for enzymatic action or energy input. C) Flexion involves the movement of the fatty acid tails of the phospholipids, which occurs spontaneously and does not require ATP. D) Fluidity is a characteristic of the membrane which refers to the overall viscosity and movement of the membrane components; it is not an individual movement of phospholipids and does not directly require ATP.
Moreover, fluidity of the cell membrane is influenced by factors such as temperature and the presence of cholesterol and unsaturated fatty acids within the membrane. This fluidity is crucial for many cellular processes including the transit of materials through the cell membrane. However, specific movements like flip-flop do require the input of energy in the form of ATP to proceed.