Final answer:
The membrane that disallows passage of Na+Cl- due to its small size is a a) 20 MWCO membrane, as it would have the smallest pores relative to the ion size and would be inhibitory to their passage.
Step-by-step explanation:
The membrane that disallows the passage of Na+Cl− is the one with a molecular weight cut-off (MWCO) that is too small to allow such ions to pass through. The MWCO of a membrane denotes the size of molecules that can pass through it, with a lower MWCO corresponding to a smaller pore size. Based on the given options and knowing that the Na+Cl− ions are small but not ultra-tiny, a 20 MWCO membrane is likely too restrictive for Na+Cl− to pass through, which means the correct answer is a. 20 MWCO. Membranes are semipermeable, which means they selectively allow certain substances to pass while blocking others depending on their chemical structure and size relative to the pore size of the membrane.