Final answer:
Plasma and interstitial fluid are almost identical chemically as they share similar compositions and are separated by a permeable capillary wall that allows for free exchange of various substances.
Step-by-step explanation:
The pair that is almost identical when compared chemically is plasma and interstitial fluid. Both plasma and interstitial fluid are very similar and are separated by the capillary wall, which is permeable to water, inorganic ions, and small organic molecules like glucose, amino acids, and urea.
However, it is not permeable to large organic molecules such as proteins. Interstitial fluid is the fluid that baths all body cells except blood cells, and it is part of the extracellular fluid which also includes plasma. Hence, the correct answer to the question is option b: Plasma and interstitial fluid.