The correct option "A. Prior to the, to" follows the correct grammatical structure for the sentence. Here's the breakdown:

1. "Prior to" is a phrase used to indicate that something happened before a particular event or time. In this case, it refers to the time before the discovery of insulin.

2. "The discovery of insulin" is a noun phrase that identifies the specific event in question.

3. "It was not possible" is the main clause of the sentence, indicating a fact about the past.

4. "To treat diabetes" is an infinitive phrase that follows "possible" and indicates the action that was not possible.

So, the complete structure "Prior to the discovery of insulin, it was not possible to treat diabetes" is grammatically correct and effectively communicates the idea that before insulin was discovered, there was no way to treat diabetes.