Why is it difficult to separate compounds that have similar molecular structures?

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Explanation:

Compounds with same molecular structures tends to have similar properties, Various separation techniques tends to use difference in properties such as solubility, melting and boiling point,  polarity, molecular interaction, and size to fail or become ineffective. This is very true in large biomolecules where the difference in composition is not sufficient to cause detectable change in properties, in fact, it is nearly impossible to prove that one molecule of biomolecular drugs, such as insulin, to be different from one another even though there are difference in molecular composition, and large scale separtion is not at all feasible.

the answer is either b or c