Assumption question: Find the conclusion. See what is essential to it. If stuck between two options, use assumption negation technique (ANT).
Let me first remind you that an assumption is a necessary missing premise. It is essential to the conclusion. If assumption becomes false, conclusion becomes false too (that is why ANT works. Negate the assumption, conclusion goes for a toss)

Conclusion: people allergic to sulfites can drink wines produced by these winemakers (those who do not add any sulfites to their wine)
without risking an allergic reaction to sulfites

i.e. if a winemaker does not add sulfites, you will not suffer from a sulfite allergic reaction by drinking his wine.
My question here is, isn't it an assumption that sulfite is not naturally present in wine even if it not added. May be grapes have it, may be during fermentation it is formed. The point is, we don't know whether it is naturally present. So if the argument says that a winemaker doesn't add sulfites so you will not suffer from a sulphites reaction, he is assuming that sulfites are not already present.
If you want to negate option E and see,
Negated (E) - Sulfites are naturally present in the wines produced by these winemakers in amounts large enough to produce an allergic reaction in someone who drinks these wines.
If this is true, our conclusion falls apart. Hence this is the assumption.

Now why is D not correct?
(D) Apart from sulfites, there are no substances commonly present in wine that give rise to an allergic reaction.

The conclusion says - You will not suffer an allergic reaction from sulfites. It doesn't say - You will not suffer any allergic reaction. Even if there are other substances that give rise to allergic reactions, you will still not get an allergic reaction from sulfites. So this is not an assumption.