For orange juice, which is usually sold in jugs or cartons and dispensed into smaller portions of uncertain volume, "some" is probably the choice that your test was looking for.

Unfortunately there are situations where "an" would be perfectly appropriate, for instance in a restaurant or if you were selecting from single serving bottles/cans of juice. To use a more common example:

Someone asks me to choose which soft drink I would like from a selection of cans, or someone asks me at a bar or restaurant what I would like to drink."I'll haveaCoca Cola."

Someone is pouring drinks from a 2 liter bottle into glasses and asks what I would like in my glass."I'll havesomeCoca Cola."

I don't want to invent a grammatical rule on the fly, but it seems as if definitely sized portions (bottles/cans/quantities served by a restaurant) use "a/an" and indefinitely sized portions (poured from a larger container, for instance) use "some

A tall glass of orange juice is the very image of ... drink is in trouble: sales of commercial orange juice are ...