GMAT Sentence Correction (SC) | SC 700 level

vailad, the mainpoint here is the isolation of insulin..and not insulin itself...
the choice which I suppose you selected (D) mistakenly refers to insulin itself in the second phrase of the sentence..which renders it incorrect.

In option (C) as per your concern, "it" is not ambigous..it correctly refers to "isolation of insulin" introduced in the first phrase...There is no mention of insulin independently hence "it" refering to just insulin does not come...

I chose