What is the principle circulating sugar in the blood and is the major energy source of the body?
What is generally used for questions where the possibilities are open, not chosen from some limited set. "What is the flight speed of an unladen swallow?"
Which, in a question, is used where what might be used, but there is some finite, well understood set of options. "The meals available tonight are steak, lasagne, or vegetable stew. Which would you like?"
In this case, either are possible, because there is a limited range of naturally-occurring sugars. However, it's quite a large range, so unless the set of sugars had been narrowed down in preceding text, I would personally tend to use what.