Final answer:
The cell membrane has a constant size, a neutral charge, and is generally insoluble to many molecules without the assistance of transport proteins. Therefore, the correct answer is 'd) Constant size, neutral charge, and insolubility'.
Step-by-step explanation:
The cell membrane is known for its selectively permeable characteristic, allowing some substances to pass through while blocking others. It has a constant size, as its overall structure does not typically change, and is essentially neutral in charge because of the balance between the phosphate heads and fatty acid tails. Molecules have variable solubility in the membrane depending on their polarity and charge; thus, the cell membrane is insoluble to many molecules without the help of transport proteins or channels. Facilitated transport is required for larger polar or charged molecules to cross the membrane, whereas very small polar molecules can cross via simple diffusion. Since the option that best fits these characteristics of the cell membrane is 'd) Constant size, neutral charge, and insolubility', this would be the correct answer to the question.